The inhomogeneous structure of crosslinked polyurethanes based on poly(butadiene)diol (PBD), 4,4’-diphenylmethane diisocyanate (MDI) and poly(oxypropylene) triol (POPT) prepared with various PBD contents were studied by dynamic mechanical analysis (DMA) and synchrotron small-angle X-ray scattering (SAXS). An inhomogeneous two-phase structure of networks containing PBD was revealed by SAXS. One phase consists of PBD chains while the other of densely crosslinked POPT/MDI network with much higher glass transition temperature. The range of inhomogeneity depends on PBD content, and is estimated as about 70-120 Angstrom from the Bragg spacing. The scattering data are fitted using the Percus-Yevick hard sphere model.