Abstract: Tantalum coating was prepared on mechanically polished thick diamond film via double glow plasma surface metallurgy technique. The surface and section morphology, micro-area composition, phase composition and bonding property of the coating were studied. The results show that the tantalum coating was continuous and uniform, consisting of tantalum metallic layer and compound layer at the interface. The coating thickness was about 1.7 μm, and the coating structure was composed of columnar crystal. A 1.1 μm thick diffusion zone with tantalum and carbon distributed in gradient was formed at the interface of the tantalum coating and the thick diamond film, and two compounds of TaC and Ta₂C were formed here. The polishing scratches on the thick diamond film were filled up by tantalum atoms, and therefore reduced the surface roughness from 128 nm to 57 nm. The obtained tantalum coating had good plasticity and good bonding performance with the thick diamond film.