This study prepared an interpenetrating polymer network of bismaleimide and polybutylene adipate-based polyurethane-crosslinked epoxy (BMI/PU-EP IPN) using the simultaneous bulk polymerization technique. Infrared spectra analysis was also performed to identify the polyurethane-crosslinked epoxy (PU-EP). Also investigated herein were the mechanical properties including tensile strength, fracture energy, and Izod impact strength of various bismaleimide content in PU-EP matrix. In addition, differential scanning calorimetry and thermogravimetric analyses of the BMI/PU-EP IPN were conducted as well. Analyses results demonstrate that the bismaleimide was dissolved primarily in the polyurethane domains of the epoxy matrix to form a compatible system, thereby increasing the mechanical strength of the BMI/PU-EP IPNs.