Polyester-based polyurethane elastomers with alkyl side groups were prepared from poly(adipate) glycols which involved 2,4-diethylpentamethylenediol or 3-methyl-pentamethylene diol as a novel glycol component and 1,5-pentamethylenediol. Segmented and allophanate-crosslinked polyurethane elastomers were prepared from these polymer glycols, 4,4＇-diphenylmethane diisocyanate and 1,4-butanediol by a prepolymer method. Mechanical properties, dynamic viscoelastic properties, glass transition temperature (T-g), and thermal hydrolytic stability were measured. The properties of the polyurethane elastomers with alkyl side groups have good mechanical properties comparable to those of general purpose polyester urethanes. Novel polyester polyurethane elastomers with alkyl side groups had improved hydrothermal stability due to steric hindrance from hydrolysis of ester groups.