Abstract: Ni-based alloy coatings were prepared by hot dipping on plain steel substrates successfully. The coated plain steel was heat treated to improve the bonding strength of interfaces between the coatings and the substrates. The microstructure and the mechanical properties of the interface were investigated before and after heat treatment. It is shown that after heat treatment, precipitates on the side of the coating are larger than those before heat treatment. Noncontinuous lump iron borides are found in the interface. The interdiffusion distance of Ni and Fe locating on both sides of the interface increases, and the transition zone evolves from 5-8 μm to 20 μm after heat treatment. There are γ-(Fe, Ni), CrB, Cr₂B, Cr₃C₂, etc. in the coatings before and after heat treatment. The transition zone is composed of three phases which are major γ-(Fe, Ni) and minor CrB and Fe₂₃(C, B)₆. From the interface to the coating, the microhardness increases gradually after heat treatment. The bonding strengths before and after heat treatment are 140 MPa and 200 MPa, respectively.