Segmented block copolyetheresters having three kinds of hard segments, i.e., poly(trimethylene terephthalate) (3GT), poly(butylene terephthalate) (4GT), poly(butylene naphthalate) (4GN) combined with poly(tetramethylene ether glycol) (PTMG) as a part of soft segment were synthesized with various hard segment contents. The number average molecular weights of PTMG selected were 1000, 2000, and 2900. The effects of hard segment content (HSC) and hard segment length (HSL) on the thermal properties of copolyetheresters were studied. Melting temperature $(T_m)$ was found to be a function of HSL for all the copolymers examined. Even at the very short HSL, 3GT and 4GN showed relatively high $T_m;while;T_m$ of 4GT decreased considerably. The 3GT hard segment showed highest hard segment crystallinity among the three, which implied that 3GT copolyetheresters had higher degree of phase separation and more stable crystalline phase in the copolyetherester elastomer than 4GT and 4GN.