Two series of thermoplastic polyurethane elastomers [poly(propylene glycol) (PPG) based PP samples and poly(oxytetramethylene)glycol (PTMG) based PT samples] were synthesized from isophorone diisocyanate (IPDI)/ 1,4-butanediol( BD)/ PPG and IPDI/BD/PTMG. The IPDI/BD based hard segments contents of polyurethane prepared in this study were 40-73 wt %. These polyurethane elastomers had a constant soft segment molecular weight (average M-n, 2000) but a variable hard segment block length (n, 3.5-17.5; average M-n, 1318-5544). Studies were made on the effects of the hard segment content on the dynamic mechanical thermal properties and elastic behaviors of polyurethane elastomers. These properties of PPG based PP and PTMG based PT samples were compared. As the hard segment contents of PP and PT samples increased, dynamic tensile modulus and alpha-type glass transition temperature (T-g) increased; however, the beta-type T-g decreased. The permanent set (%) increased with increasing hard segment content and successive maximum elongation. The permanent set of the PT sample was lower than that of the PP sample at the same hard segment content.