The Q235A mild steel and AISI304 austenite stainless steel were subjected to solid diffusion welding by vacuum diffusion bonding approach to investigate the influence of welding temperature on the interfacial morphology, microstructural constituents and mechanical properties. The results show that the single ferrite layer (zone II) and carbon-enriched layer (zone III) were formed nearby the bonding interface of Q235A mild steel and AISI304 austenite stainless steel, and heterogeneous microstructure on both sides of interface formed a common grain boundary by diffusion. The strength and toughness of the bonded joint reached the highest values, for welding temperature of approximately 850 ℃, welding pressure of beyond 10 MPa, and welding time of approximately 60 min, which was larger than those of the Q235A mild steel layer. Otherwise, the Cr₂₃C₆ carbide easily formed at a relatively lower temperature (≤800 ℃), whereas the secondary carbides and intermetallic compounds formed at a relatively higher temperature (≥900 ℃). Both cases would dramatically deteriorate the strength-toughness of the bonded joint. Therefore, it was proposed that the brittle precipitate phases can be effectively avoided by controlling the welding temperature to approximately 850 ℃, thus ensuring the resulting performance of the bonded joint.