The work aims to discuss the variation of microstructure and hardness of nanocrystalline NiCrC coating after long-term heat treatment. The coating was prepared by high velocity air-fuel (HVAF) spraying with cryomilled nanocrystalline NiCrC powder as the feedstock and then heated at 650 ℃ for up to 200 h. The microstructure, phase constitution, grain size and hardness of coating were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis and a Vickers microhardness tester. In addition, the nanocrystalline NiCrC powder was also studied for comparison. The microstructure of coating was characterized by a nanocrystalline metallic matrix with dispersed small carbide particles. During long-term heat treatment, recrystallization and grain growth occurred in the coating material, together with exsolution of metal matrix and the precipitation, phase transformation and growth of the carbides. The NiCrC coating exhibited good thermal stability. The average grain size grew up to a relatively stable length of 100 nm from the original size of 41 nm after heat treatment for 50 h. The coating hardness increased to the maximum value of 801HV300 (15 s) from the original hardness of 697HV300(15 s) and then dropped to a relatively stable number of 729HV300(15 s) after heat treatment. The nanocrystalline NiCrC powder possessed the similar variation trend of microstructure and hardness with the coating. The exsolution and grain growth of metallic phase cause the softening of nanocrystalline NiCrC coating matrix during thermal exposure at 650 ℃. However, the precipitation of fine carbide particles and the carbide content increasing resulted from phase transformation (Cr7C3→Cr23C6) compensate the matrix hardness softening, and further lead to the increase in the overall coating hardness.