Two series of flexible polyurethane foams were fabricated by substituting conventional petroleum-based polyols with increasing amounts of soy-based polyols (SBP) having different hydroxyl numbers. The mechanical properties of the foam. were characterized by stress-strain analysis in the compression mode and DMA in tension mode, the cellular morphology was analyzed by SEM and the microphase-separation of the foams was noted by SAXS. Our results showed that the Cellular morphology and mechanical properties of the flexible foams were affected significantly by the foam fabrication method and SBP hydroxyl numbers. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 112: 299-308, 2009