Although girls achieve equal and often better school results in mathematics and natural sciences, they still choose STEM educational tracks to a lesser degree and are underrepresented in most STEM occupations. It is thought that this difference in interest for STEM and the difference in representation of women and men in various STEM occupations might be in part explained by the gender stereotypical belief that STEM is for boys and that boys are naturally more gifted for STEM. There is no research that looks at both the endorsement of gender stereotypical beliefs of seventh-graders regarding STEM schooling and the workplace, nor various STEM fields. Therefore, the aim of this research was to examine the endorsement of gender stereotypical beliefs in parents and children regarding STEM as a typically “male” field and bring such beliefs in relation to children’s motivation towards STEM. In addition, the efficiency of a single, general STEM intervention program was evaluated with regard to decreasing the gender stereotypical beliefs of students and their connection to gender stereotypical beliefs of parents. The research is set within the Eccles et al. (1983) expectancyvalue theory framework which was used to explain the role of gender stereotypical beliefs of children and parents regarding motivation of girls and boys towards STEM. Cross-section research was conducted using a posttest-only control group quasiexperimental design. A convenient sample of 842 seventh grade students from 16 elementary schools from Zagreb and the surrounding area and their mothers or female guardians participated in the study. Students from eight schools belonged to the treatment group and students from the remaining eight schools belonged to the control group. The students were tested in groups during regular classes in their classrooms in the duration of two school hours, while the testing of the parents was conducted via questionnaires that were delivered to the parents in sealed envelopes that they returned to the school after completing the questionnaires. The students, among other measures, filled out various self-assessment forms of their own gender stereotypical beliefs and motivation for various STEM education areas and workplace fields, while the parents self-assessed their own gender stereotypical beliefs regarding STEM. All the used instruments proved to have good metric qualities, i.e. high reliability and expected factor structure in the overall sample, as well as on the sample of the control and treatment schools, and the sample of boys and girls. The STEM intervention program was conducted in eight treatment schools during two consecutive school years, across five encounters in the total duration of 13 school hours. The program included multidisciplinary workshops organized in the schools and visits to the Ruđer Bošković Institute, as well as visits to various laboratories and institutes of the Faculty of Electrical Engineering and Computing. The main goal of the intervention was to increase the motivation of students for STEM areas; therefore, the intervention activities were based on interactive teaching, practical work and interaction with their peers using equipment and materials the students found interesting. The students were presented with new content regarding STEM that they did not have the chance to see in their regular schooling (e.g. electronics, robotics and the like). The primary focus was on providing as much information as possible about various occupations and careers in STEM by visiting institutions for science and higher learning related to STEM areas. The students got the chance to get insight into the wide range of STEM workplaces and work-tasks through lectures, experiments, tours of facilities, laboratories and institutes, as well as through talks and questions posed to STEM experts. The results showed that students, as well as their mothers, consider STEM to be an area more suited for boys, with beliefs more pronounced related to STEM occupations, as opposed to STEM education. Beliefs were also more pronounced related to the field of engineering and technology, as opposed to natural sciences and mathematics. The research did not confirm the mediation role of children’s gender stereotypical beliefs in relation to parents’ gender stereotypical beliefs and the children’s motivational outcomes. However, the results showed that the stronger gender stereotypical beliefs of mothers positively influenced the motivational outcome of boys and facilitated more pronounced gender stereotypical beliefs of girls. Simultaneously, stronger STEM related gender stereotypical beliefs of children positively influenced the motivational outcomes of boys and negatively influenced the motivational outcomes of girls. The obtained results help in understanding the motivation of boys and girls for STEM areas and point to the need for creating intervention programs that would be specifically targeted at mitigating gender stereotypical beliefs in order for students to make future educational choices regardless of gender stereotypical beliefs.