The relative formation rates of castor oil polyurethane and acrylic or vinyl polymer during formation of the simultaneous grafted interpenetrating polymer networks (IPN) were compared by using IR spectrophotometry. The relation between gelation point and gel content of IPNs with different compositions were determined. Tensile strength of the IPNs containing different acrylic or vinyl polymers varies with polyurethane content and kind of acrylic or vinyl monomer. TEM of the IPN indicated that polyurethane existed as domains in acrylic polymer. A wide glass transition temperature of the IPN occurred in the dynamic mechanical spectrum of the IPN.