Abstract: Stellite 6 coating was sprayed on 45 steel substrate by electrothermal explosion directional spraying with two sizes of stellite 6 alloy. Thickness uniformity, porosity, microstructure, grain size and bonding between substrate and coating were analyzed with scanning electron microscope, optical microscope and image analysis software respectively. Hardness of the coating was tested. Comparisons of coatings sprayed by different sizes of the alloy were carried out.The results show that grains of coatings are greatly refined for both coatings. Porosity is very low for two coatings. The thickness of coating sprayed by small size of the alloy is more uniform than that of coating with larger size of the alloy. There is a good bond between substrate and coating because diffusion occurs in the interface. Hardness of both coatings are far higher than that of stellite 6 alloy. The maximum hardness of the coating is up to 997 HV for small size of sprayed coating and 738 HV for larger size of sprayed coating, 2-3 times of the sprayed material.