Abstract: Two micro-nanostructured and a conventional WC-12Co cermet coatings were deposited by High Velocity Oxy-Fuel spray (HVOF), and the morphologies and structures of the coatings were analyzed by SEM, XRD. Porosity, microhardness and fracture toughness of the coatings were measured. Resistance of coatings to cavitation erosion was studied by ultrasonic vibration cavitation equipment and the cavitation mechanisms were explored. Results show that for the micro-nanastructured WC-12Co coating composed of submicro and nano WC had the lowest porosity and the densest microstructure in comparison with other two WC-12Co coatings. Decarburization of WC occurred in some degree and W₂C, W and Co₆W₆C phases formed during spraying micro-nanostructured WC-12Co powders although obvious decarburization was not observed for conventional WC-12Co powder. It is found that the micro-nanostructured WC-12Co coating made up of submicro and nano WC exhibited the best cavitation erosion resistance and the erosion rate was approximately one third that of conventional one. The enhanced cavitation resistance of the micro-nanostructured WC-12Co coating was attributed to the presence of submicro and nano sized, high microhardness WC, W₂C and Co₆W₆C particles and tough Co and W in the micro-nanostructured HVOF WC-12Co coating. This increased the coating microhardness and fracture toughness and made it difficult for the formation and propagation of the microcracks.