Dynamic scanning calorimetry analysis (DSC), combined with other complex investigations was performed in order to test the thermal stability and to evaluate the structure–properties relation of the developed composites. The contribution of additive to the protection of PLA against oxidative degradation is discussed in respect with phenolic compounds contained by rosemary powder. Polylactic acid (PLA) is nontoxic, biodegradable, bio-absorbable, compostable polymer obtained from renewable resources that could be thermally processed and finds utilization in a variety of fields. In order to maximize the benefits and versatility of PLA, it is necessary to understand and combine the relationship between the properties of polyester matrix and characteristics of dispersed phases (additives, etc.), their compatibility and interactions, stabilizing or degradation effects, influences of manufacturing process on the characteristics of final products, and so on. These influences are evidenced in DSC curves which are shifted to higher temperatures by natural antioxidant incorporation – Fig.1. Fig. 1. DSC curves of PLA and PEG-plasticized PLA containing bioactive agents as chitosan as antimicrobial and rosemary extract as antioxidant The results were corelated with those obtained by other methods such as mechanical tests, rheology, ATR-FTIR, and SEM.