Nowadays, advancement in techonoly brought development of 3D printers which allows printing the object without additional processing. Material which is used the most in 3D printing is powder of Polyamide 12 (PA 12). Purpose of this work is to examine influence of SLS process on structure and properties of PA 12 powder. Three generations of PA 12 powder were analyzed: samples of virgin powder, samples which were used two times in SLS process and samples of waste powder. Furthermore, powder samples were pre-melted in Brabender to investigate the possible further use of waste powder. Thermal properties of samples were determined by differential scanning calorimetry (DSC), and thermal stability by thermogravimetric analysis (TGA). Mechanical properties were determined by tensile test and particle size distribution of powder by laser diffraction. The results of the DSC analysis showed that by increasing the number of SLS cycles, the arrangement of the crystal structure in powder decreases, which is a consequence of the melting of one part of the crystal lamellae due to the high temperatures in the SLS process. Also, with melting there is partial lost of the crystalline structure arrangement and significant reduction in degree of crystallinity. TGA analysis showed decreasing in initial thermal stability, but increasing stability at the end of degradation for examined powder samples as PA powder has undergone multiple cycle of 3D printing. Same trend is also in samples which were pre-melted. Tensile test indicated that there was not significant changes of strain and elongation in yield point, but with increasing number of SLS cycles of process there is increase of secant modulus. Dissipation of values for strain and elongation at break presents major limit for using waste powder in for further applications.