The basic idea of the reactive plasma spray (RPS) was presented. The TiN coating was successfully deposited on Q235 steel substrate by using the gas_tunnel plasma spray gun via RPS. Micro_hardness of the TiN coating under different loads has been measured, revealing an obvious indentation size effect, and a weak work_hardening effect. XRD, TEM and HRTEM were employed to analyze the prepared coating samples. Results obtained show that nanocrystalline TiN coating has been achieved via RPS, which is composed of TiN crystals of about 57—70nm in diameter and the amorphous TiN.