The aim of this study was to research the influence of polyethylene (PE), which is added in various amounts, on the thermal and mechanical properties, also, morphology of polymer blends based on polylactide (PLA). The purpose of blending PE with PLA was to produce a biodegradable polymer blends with a small amount of petroleum – based synthetic component and to reduce the price of final polymer product. As the second component in the PLA – based blends, polyethylenes of various densities were used, such as low density and high density polyethylene. Furthermore, from the point of application of the final polymeric product, the aim was to overcome the disadvantages of PLA, such as high modulus and stiffness, while maintaining good properties of PLA, such as high strength, to obtain firm, but elastic and flexible material. Due to the immiscibility of the components, it is necessary to add filler with the purpose to modify the interface between polymers which would provide better adhesion and mechanical properties of the material. In order to study the effect of filler on the PLA/LDPE and PLA/HDPE systems CaCO3 nanofiller (5wt. %) was added to the selected blends, and the properties of nanocomposites were tested. Pure components, blends and composites were characterized by tensile test and impact resistance examination, differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). When the second component is added, it reduces the initial high strength at break of pure PLA and, also, module and stiffness, while the elasticity increases only with larger amounts of PE in blends. The worst properties for both systems, with LDPE and HDPE, are noticed for the mixtures with the same amounts of both polymers. Nanofiller was added to the mixtures with small amount of PE component. Nanofiller is not situated at the interface, but it still affects the properties of PLA/LDPE blends. Nanofiller improves strength and toughness; also it increases crystallinity of PLA/LDPE systems, but has no effect on mechanical properties of PLA/HDPE blends.