The diffusion bonding process, microstructure and mechanical performance of low-carbon reduced activation ferritic/martensitic (RAFM)steel were studied. It indicated that the increase in bonding temperature, pressure and time would be beneficial to refine the microstructure of the bonded joint. The microstructure after diffusion bonding was examined using SEM and TEM. Reliable diffusion bonded joint of large-sized low-carbon RAFM steels was obtained in a vacuum diffusion bonding equipment. The fracture all took place in the base materials during tensile tests, and the joint exhibited higher tensile strength than parent materials. The martensitic lath close to the joint was finer than that in the matrix, and the M23C6 carbides were precipitated densely along packet and lath boundaries upon the subsequent heat treatment. In addition, the joint exhibited excellent impact performance.