WC-Co coatings are widely used for sink rolls in a continuous galvanizing line. They can extend the service life of the rolls from being attacked and improve the quality of galvanized steel sheets. Recent developed nano-structured WC-Co coatings exhibit better material properties than conventional micro-structured ones. In this study, a high velocity oxy-fuel (HVOF) spray gun was used to prepare nano-structured and conventional WC-Co coatings. Material properties and their corrosion resistances in molten zinc were evaluated. The results showed that the nano-structured WC-Co coating had a higher hardness, a lower porosity and a better toughness. However, the conventional WC-Co coating exhibited better ability against molten zinc corrosion. The main mechanism of molten zinc corrosion is the inter-diffusion of molten zinc and Co. When Co diffuses into the molten zinc, nano-sized WC particles would become easier to detach from the coating than micro-sized ones.