Modification of polylactid acid (PLA) properties was performed by addition of magnesium hydroxide (Mg(OH) 2 ). Before preparation of the composites, milled granules of PLA delivered by the supplier had to be cleaned because they contained visible impurities. Using the differential scanning calorimetry (DSC) it was determined that the impurities are mainly made of high density polyethylene (PE-HD). Removal of the impurities from the polymer was not successful. So, PLA/Mg(OH) 2 composites had to be prepared from the PLA granules in the laboratory single-screw extruder. Despite the fact that in previous studies processing procedure of PLA granules has been the biggest issue, composites with satisfying homogeneity were made. Except for the determination of different types of impurities, DSC was also used to study the thermal properties of PLA/Mg(OH) 2 composites. Addition of inorganic filler has a complex effect on thermal properties of PLA. Among other things, it is clear that inorganic filler reduces PLA crystallinity. Mg(OH) 2 does not work like a nucleating agent in PLA composites. Content of Mg(OH) 2 from 15 to 20% represents the area of different Mg(OH) 2 impact on thermal properties of PLA. The research of thermal stability was investigated by the thermogravimetric analyses (TG). Degradation of PLA and Mg(OH) 2 includes only one degradation step while degradation of PLA/Mg(OH) 2 composites have up to five degradation steps. With increase of Mg(OH) 2 content in composites their thermal degradation becomes more complex and their thermal stability is getting worse.