In this paper, the density of waste particles and the specific gravity of particles of mechanically and biologically treated waste from the waste management center Marišćina were studied. In this research, waste components were separated manually to obtain the most accurate value of individual waste components, since waste changes its properties and is not homogeneous. The water pycnometer procedure was adapted from BS 1377: Part 2: 1990 standard for the determination of particle density with a simple Large water pycnometer (LWP) method. A 1000 mL pycnometer was used and samples varied depending on the component which was studied. Representative samples of 31 g were taken for paper and cardboard, 14.7 g for plastic and 75.9 g for glass. Plastic varied 13.52%, glass 5.27%, and paper and cardboard 5.65% in relation to the mean particle density for each component. The obtained results indicate that glass in this study is natural glass, because its values are within the values for natural glass which can be found in literature, while plastic and paper had slightly lower values of mean specific density than those found in literature. Thus, plastic in this study had a mean specific density 10% lower than HDPE and LDPE plastics, and 8,82% from PP plastics found in literature. The specific density of paper was 7,5% lower than the value of the specific density found in literature.