The possibility of phase separation of benzoxazine (BZ)/epoxy resin (ER) blends were investigated via model components. The experiment result indicated that if the molecular weight of ER can increase to a higher extent (for example, ⩾4370 g mol−1) before the polymerization of BZ resin occurs, phase separation of BZ/ER blends can take place in-situ. By varying the ER content in the system, sea-island structure, ribbon weave-like bi- continuous structure and inverted-phase structure can be obtained.