Relation between structures and properties of polyurethaneurea synthesized from polytetramethyleneglycol (PTG), diphenylmethanediisocyanate (MDI), and ethylenediamine was investigated. Average chain length of softsegment and hardsegment was simulated stastically based on the measurement of unreacted free MDI in prepolymer. Influence of chemical structures such as the length of softsegment, hard segment and PTG, the molecular weight distribution of PTG, and the molecularweight of polyurethaneurea on the elasticity was studied. The importance of low melting point of softsegment and small hard domain size were appointed to realize good elasticity.