The relationship between morphology and mechanical properties were investigated in a binary blend of photocurable polymer (2,2-bis(4(acryloxy diethoxy)phenyl)propane; BPE4) and linear polymer (polysulfone; PSU). The blend films are prepared by in situ photopolymerization of homogeneous mixtures of BPE4-monomer and PSU. The tensile strength and modulus reach a maximum in the blends having vague domain structures with a diffusion-phase boundary, at which the underdeveloped small BPE4-rich domains are wrapped with the PSU-rich matrix. The higher strength and modulus are caused by a strong interfacial adhesion between the BPE4-rich domain and the PSU-rich phase, which is brought about by incomplete phase separation in the PSU-rich phase. The tensile strength and modulus decrease as the bicontinuous structures clear due to deterioration of the interfacial adhesion. The mechanical properties also deteriorate in the blends with semi-IPN structures due to the lower conversion of BPE4. The composition dependencies on the tensile strength and the modulus for blends cured at optimum cure temperatures were found to give convex curves reaching maximums greater than those of pure components.