Polylactide (PLA) is a commercial biodegradable thermoplastic material which can be obtained from biosources, because of that PLA is potential ecological alternative to petroleum-based polymers. Its application is limited by its low toughness, which can be increased by mixing PLA with thermoplastic polyurethane (TPU). TPU is an elastic polymer with good mechanical properties. Samples of the neat PLA and TPU and PLA/TPU blends with different content (80/20, 60/40, 50/50, 40/60, 20/80) were prepared in the Brabender mixer. Thermal stability of the neat PLA and TPU as well as PLA/TPU blends was investigated by thermogravimetric analysis (TGA), while viscoelastic properties were determined by dynamic mechanical analysis (DMA). The morphological structure of the neat polymers and PLA/TPU blends was obtained by X-ray diffraction analysis. The resistance of the samples to the distilled water was determined by the swelling test. Results of TGA measurements confirmed that the neat PLA is thermally more stable than the neat TPU because of the higher decomposition temperature. Thermal stability of PLA/TPU blends in the nitrogen atmosphere decreased with addition of TPU. According to DMA results the PLA/TPU blends are partially miscible. The swelling test revealed that PLA, TPU and PLA/TPU blends are hydrophobic materials and that slightly swell in distilled water. Results of XRD analysis showed that PLA has an amorphous structure, while TPU has amorphous and crystalline phases in the structure.