The diffusion bonding has been applied to the dissimilar metal joining of aluminum bronze (JIS C-6191) to austenitic stainless steel (JIS SUS 316L) which is very difficult to bond by conventional welding techniques. The microstructure of the bond zone and fractured surface of the joints have been examined to get better understanding of important factors that determine the strength of the joints. Results obtained are summarized as follows.In the diffusion bonding without insert-metal the maximum tensile strength of the joint, which was obtained at the bonding temperature of 1273K, was about 300MPa much lower than the strength of base metals. In this joint the stainless steel adjacent to the bonding interface undertook transformation from austenite to ferrite owing to the increase in the concentration of aluminum (ferrite former element) supplied from aluminum bronze. Along the grain boundaries in the transformed layer, an intermetallic compound composed of Al, Cu and Ni was observed. When the joint bonded at 1273K was subjected to a tensile test, crack propagated chiefly along the intermetallic compound. Therefore, the intermetallic compound was considered to be an important factor that depressed the strength of the joint.In order to improve the strength of the joints, copper foil and iron foil as the insert-metal were used. The formation of the intermetallic compound was suppressed considerably by using the insert-metal. However, when iron insert-metal was applied, it was very difficult to attain intimate contact between stainless steel and the insert-metal, because the flow stress of the insert-metal was much higher than that of aluminum bronze at bonding temperatures adopted. On the other hand, when copper insert-metal was applied. The tensile strength of the joint was considerably increased compared with that of the joint without insert-metal. The maximum tensile strength of 400MPa was obtained at the bonding temperature of 1313 K by using the copper insert-metal 50 μm thick.