During the tensile testing of thermoplastic elastomers it is difficult to obtain true stress-strain curves because of necking phenomenon. Thus, we conducted a high-speed tensile testing of Thermoplastic polyester elastomer's (TPC) notched specimens with strain measurement using digital correlation and reported the true stress-strain relations under various tensile speeds in this study. Because notched specimens enable control necking position, TPCs vary depending on the composition and composition ratio of the basic composition of the hard and soft segments. TPCs with two different hardness values (D52 and D57; ASTM D2240) were investigated. In typical TPCs using in this study, the hard and soft segments are composed of a polybutylene terephthalate chain and a polytetramethylene glycol chain, respectively. Because the strain rate and yield stress relationship are known to strongly influence the internal structure of TPCs, the relaxation time behavior depends TPCs' internal structure. In this study, we investigated the strain speed dependence in a TPCs. From the results, we could discuss quantitatively about influence on mechanical properties by changes in the internal structure depended on hard segment and soft segment in necking. POLYM. ENG. SCI., 58:E151-E157, 2018. (c) 2018 Society of Plastics Engineers