A novel modified polybenzoxazine with bi-continuous phase separated morphology was successfully prepared from the blend of bisphenol A-aniline benzoxazine (BA-a), N,N′-(2,2,4-trimethylhexane-1,6-diyl)bismaleimide (TBMI) and catalyst imidazole. The thermal and toughness properties of the phase-separated blend cast were improved compared with polybenzoxazine.