The mechanical properties of urethane elastomer prepared from hydroxy terminated polybutadiene(HTPB). trimethylhexanediol(TMHD), and hexamethylendiisocyanate(HDI) were studied. Ab the concentration of chain extender was increased, the value of tensile strength, modulus, and hardness were enhanced due to the increase in the number of polar urethane groups, while the rupture strain was decreased. The enhanced mechanical properties for samples prepared from 1,6-HD were found to be stemmed from the crystallinity in the elastomer and from the hydrogen bond which acts as a hard segment. The crystallinity was confirmed by X-ray diffraction and DSC. The molecular weight between crosslinks of elastomer were also investigated from the Mooney-Rivlin equation. The phantom theory gave better agreement than affin''s with the experimental data.