The SDC99 steel is a new kinds of cold work die steel with high strength, high wear resistant and toughness. However, in order to extend its service life, the surface modification technology is required. Many surface treatments can be used to improve the wear resistant of cold work die steel. Among these methods, transition metal nitrides and carbides (such as VC) fabricated by TD (thermal diffusion carbide coating process) have been commonly used in tribological applications to enhance the life of mechanical components because they possess high hardness, excellent wear resistance, low coefficient of friction and good corrosion resistance. During the TD process, materials are immersed in to a molten borax bath that contains the relevant carbide/nitride forming element such as V, Nb, Ta, Cr, Mo and W, and the coating is formed by the combination of metal elements as mention above with the carbon/nitrogen diffused from the substrate. In order to investigate the growth mechanism of VC coating on SDC99 steel, salt bath vanadizing by TD process were performed by molten borax bath containing Na₂B₄O₇(75.6%),BaCl₂(8.4%), V₂O₅(10%) and Al powders(6%). The experiments were carried at 850, 900, 950,1000 and 1050 ℃ for various time. The microstructure and morphology of VC coating layer were observed using SEM and metallographic measurements, and the elemental analysis on the surface were carried out with EDS. The crystalline structures of samples were characterized with XRD.The hardness gradient of nanoindentation test was performed on Triboindenter in-situ nanomechanical test system. The results indicated that at the same immersion time, the higher the temperature of salt bath was, the thicker of prepared coating was. The VC coating layer possesses much greater hardness (about 22 GPa) than that of the substrate (about 7.0 GPa). The hardness between VC coating layer and substrate decreases slowly, indicating that the microstructure of substrate adjacent to the interface can provide effective supporting for VC coating layer. The growth mechanism was the nucleation and growth of initial grains, and the formation of sub--micron sized grains on them. Moreover, the crystalline grain of coating grew into equiaxed grains, due to the influence of carbon activity in substrate, and the grain size decreased with the decrement of its distance to the substrate. In addition, at the initial growth stage of crystalline grain of coating, the growth preferred orientation of crystalline grains on SDC99 steel transformed from crystal plane (200) to (111). However, with the increment of time, the growth preferred orientation of crystalline grains was unsignificance and they grew into equiaxed grains.