A WC-17Co coating was deposited onto ST37 mild steel substrate using HVOF spray technique and then heat treated at different temperatures in a vacuum chamber. The coatings were then evaluated in the as sprayed and heat treated conditions. The results indicated that substantial amount of amorphous phases was created during the HVOF spray process. Carbon and tungsten, liberated through the dissociation of WC, combined with cobalt in the spray powder to form amorphous material on solidification. X-ray analyses indicated that by increasing the heat treating temperature, the percentage of the desirable crystalline phases was increased and the less desirable amorphous phases were eliminated. Cross-sectional microhardness examination showed significant increases in the hardness of the coating after heat treatment. The residual stress of thermally sprayed WC-17Co deposits before and after heat treatment was also studied using X-ray diffraction technique. The heat treated coatings exhibited residual compressive stresses to varying degrees. The stress decreased in the carbide grains after the relaxation occurred by heat treatment.