In this study three polyurethane (PU) flexible foams were prepared by using poly ether polyol and by replacing 30% of the polyether polyol with soybean oil-derived polyol (SBOP) and styrene acrylonitrile (SAN) copolymer filled polyol. Scanning electron microscope, dynamic mechanical analysis (DMA), and small angle X-ray scattering (SAXS) were used to examine the cellular structure and the polymer morphology. Compared to the all petroleum-based PU foams, DMA of the SBOP-containing foam showed broader glass transition temperature accompanied by a more gradual decay of the storage modulus (G′) with temperature. The SAXS results exhibited a decrease in signal intensity as SBOP is incorporated into the foam, which corresponds to a decrease in electron density contrast between the hard and soft phases. Both behaviors observed in DMA and SAXS suggest an increased degree of phase mixing in SBOP-containing foam.