In accordance with the need for fast processing, development, quality of polyamide products and environmental friendliness of their processing techniques, 3D printing technology has been attracting special attention for several years. The main advantage of 3D printing technology is the fact that in a very short time complex products can be produced using a wide range of materials (polymers, metal, glass). One of the possible techniques of 3D printing is selective laser sintering (SLS). The principle of the SLS method is based on bonding the powder material using a CO2 laser to the previously applied layers. A very large proportion of polymeric materials applied by the SLS method are based on polyamide 12 (PA12). During the heating process, which is a necessary part of the SLS method, PA12 powder loses on the quality of its mechanical and thermal properties, therefore there is a limited number of cycles of its use. After that it becomes waste. In order to reduce the amount of PA12 waste generated in the mentioned method and to enable its reuse, the material is often mixed with reinforcements which affect the improvement of the properties disturbed by the heating process. The aim of this work was to prepare composites by mixing waste PA12 and glass fibers and to examine their surface, thermal and mechanical properties. The starting polymer PA12 and PA12 / SV composites with a glass fiber content of 0.5%, 1.0%, 3.0% and 5.0% by weight were prepared by mixing in a Brabender kneader. Surface characteristics were tested on the samples by the contact angle method, thermal properties using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), and mechanical properties using tensile testing. The calculation of the adhesion parameters was used to evaluate the polymer / reinforcement interactions. The results indicate the possibility of establishing significant polar interactions between glass fibers and PA12. Thermal analysis showed that the addition of glass fibers reduces the mobility of the polymer matrix chains while the order of the crystal phase remains the same for all composites. The addition of fibers interferes with crystallization. The addition of glass fibers does not affect the onset of PA12 degradation but the end of degradation is shifted toward higher temperatures. In the case of mechanical properties, the influence of glass fibers on the change in elongation while maintaining high strength is visible, which indicates good adhesion between PA12 and glass fibers.