Nowadays new compounds and systems based on biopolymers have taken an advantage in the development of medical materials. Therefore, significant advances have been made in the preparation of implants, blood contacting devices, sensors or artificial organs. Because of unique physicochemical and mechanical properties multiblock thermoplastic elastomers (TPEs) are used in a wide range of medical applications. They show good solvent resistance, elasticity, tear strength, and their properties can be modified by different hard/soft segments ratio. One of important biofunctional feature is their transparency if used in blood contacting devices (for visual inspection of blood clot formation). This feature can be found in thermoplastic polyurethanes known from decades. A big challenge is to produce transparent polyester-based thermoplastic elastomers since they are known to have high tendency to crystallize. In the present work, segmented TPEs containing hard segments as in poly(ethylene terephthalate) (PET) and soft segments comprising amorphous fatty acid ester sequences based on dilinoleic acid (DLA) were synthesized with the aim to obtain transparent materials with high strength and flexibility. Thermal properties of the obtained PET/DLA copolymers were examined using differential scanning calorimetry (DSC). Mechanical properties, such as Young modulus, tensile strength and ultimate elongation were studied as the function of hard/soft segment ratio. The influence of processing parameters on materials transparency has been investigated in details indicating the importance of type of catalyst used for the synthesis of PET/DLA copolymers. Acknowledgements This work has been financially supported from the National Centre for Research and Development in a frame of “ElastoKard” project.