Uniform copolymer films of benzoxazine resin (BA-a) and urethane prepolymer (PU) were prepared at various BA-a/PU mass ratios (100/0, 80/20, 60/40, 40/60, 20/80, and 0/100) via sequential cure method comprising of moisture cure and thermal cure steps. In the moisture cure step, Fourier Transform Infrared (FT-IR) spectra revealed the network formation between NCOterminated group and moisture to firstly produce PU solid film. Then in the thermal cure step, the change of tri-substituted benzene ring to tetra-substituted benzene ring was observed suggesting polybenzoxazine network formation in this step. Moreover, the spectra reveal that isocyanate groups in polyurethane structure could react with phenolic hydroxyl groups of BA-a to form biuret and allophanate groups. Dynamic mechanical analysis (DMA) confirms a synergistic behavior in glass transition temperature (T g ) of the alloys with the highest T g value of 275 C which is uniquely observed in these alloys obtained from traditionally thermal cure method. The proposed sequential cure method above is found to be highly useful for uniform coating or film casting process which lacks in traditional, low A-stage viscosity, benzoxazine resin.