Primary amine-containing benzoxazines 3 and 6 were prepared from an unsymmetrical diamine 1, in which the amine group ortho to diethyl substituents, the hindered amine, is not reactive toward aldehyde at room temperature. Based on 6, three poly(benzoxazine imide) (PBzI) thermosets were prepared by low-temperature polymerization, followed by thermal treatment. Experimental data show that incorporating thermosetting benzoxazine into a polyimide increased the T-g, tensile modulus, dimensional stability, flame retardancy, and contact angle without sacrificing too much in thermal stability.