Graft-interpenetrating polymer networks (graft-IPNs) of polyurethane (PU) and bismaleimide (BMI) were prepared by using a simultaneous polymerization technique. The effects of the PU molecular weight and the amounts of the PU on the mechanical properties, thermal stability, dynamic mechanical properties, density, and morphology of the IPNs are discussed. The graft-IPNs exhibited superior ultimate tensile strength and large improvement in flexural strength when the short soft segments of the PU were introduced. Better thermal stability of the IPNs was shown by increasing the BMI content. The heterogeneous and phase-separated characteristics were found from the results of dynamic mechanical analysis. The maximum density was achieved by the graft-IPNs structure when 33.3 wt % of PU with short soft chain length was added. The graft-IPNs also showed heterogeneous morphology in the micrographs when a large amount of PU was incorporated.