Thermoplastic polyurethane elastomers were prepared from 4,4-diphenylmethane diisocyanate (MDI)/1,4-butanediol (BD)/poly(propylene glycol) (PPG) and MDI/BD/poly(oxytetramethylene glycol) (PTMG). The MDI/BD-based hard-segment content of polyurethane prepared in this study was of 39-65 wt %. These polyurethane elastomers had a constant soft-segment molecular weight (<(M-n)over bar>, 2000), but a variable hard-segment block length (n, 3.0-10.1; <(M-n)over bar>, 1020-3434). The effects of the hard-segment content on the thermal properties and elastic behavior were investigated. These properties of the PPG-based MPP samples and the PTMG-based MPT samples were compared. The polyurethane prepared in this study had a hard-segment crystalline melting temperature in the range of 185.5-236.5 degrees C. With increasing hard-segment content, the dynamic storage modulus and glass transition temperature increased in both the MPP and MPT samples. The permanent set (%) increased with increasing hard-segment content and successive maximum elongation. The permanent set (%) of the MPP samples was higher than that of MPT samples at the same hard-segment content. The value of K (area of the hydrogen-bonded carbonyl group/area of the free carbonyl group) increased with increasing hard-segment content in both the MPP and MPT samples, and the K value of the MPT samples was higher than that of the MPP samples at the same hard-segment content.