The mechanical deformation and microstructure formation at the interface of a high velocity oxygen fuel (HVOF) sprayed WC-Co splat on a carbon steel (0.45%C) have been studied by the focused ion beam (FIB) cross-sectioning and examined by scanning electron microscopy (SEM). Several types of WC-12wt.%Co powders with different WC particle sizes, 0.2 ~ 7.0µm were used for spraying to investigate the size effect of these WC particles onto the splat morphology and microstructure around the interface. The interface fracture toughness of the coatings made of these powders was evaluated by the pre-notched four-point bending test and the correlation between the splat microstructure and the toughness variation was investigated. The interface fracture toughness of these WC-12%Co coating / carbon steel was evaluated as about 600~1700J/m2. It was revealed that the WC particle size in the original powders has large effects onto the interface adhesion.