The polyurethanes composed of methylene bis(4-phenylisocyanate) (MDI) and 1,4-butanediol as the hard segment and hydroxyl-terminated poly(ethylene adipate) as the soft segment were synthesized with varying the hard segment contents (15-40 wt.%) and investigated by differential scanning calorimetry and dynamic mechanical analyser. In the DSC thermogram two endotherm peaks were observed above the glass transition temperature of the hard segment. The experimental results for the different annealing time and heating rate in DSC showed that there exsisted two different crystal forms in the hard segment of the polyurethane. From I crystal Melts at 160℃ and form Ⅱ crystal melting at 190℃ is thermodynamically more stable than form I. The endotherm peak of form Ⅱ was always increased with falling off the endotherm peak of form I by annealing. These results were well correlated with the dynamic mechanical data by showing the dramatic change in dynamic shear modulus and a trough of loss tangent around 160℃ caused by form I.