WC-12Co coatings were sprayed by HVOF with a composite mixture of microstructure and nanostructure powder. The thickness of the coatings are 0.5mm, 1mm, 2mm, 3mm and 4mm respectively.SEM and XRD were used to analyse the microstructure and phase composition of the coatings. Porosity of the coatings was also tested. Results show that thickness has little influence on surface microstrucure and roughness of the coating, on the contrary, thickness has great influence on section microstrucure. For coating which is thicker than 1mm, microstructure of the middle part of the coating is denser than the interfacial and the upper area, however, microstructure changes litte along the deposition direction for coating which is thinner than 1mm. There are few W2C and Co3W3C phase in the coating for thermal decomposition of WC, furthermore, more and more decompostion occurs with the increase of the coating thickness.