The effect of pack boronizing on microstructure and hardness of WC-12wt%Co coating sprayed on a low carbon steel (SS400) was studied using two kinds of HVOF-sprayed WC-Co coatings consisting of a single phase of WC and several phases of WC, W2C and Co3W3C, respectively. Pack boronizing was applied at 1273K for 3.6ks under an argon flow atmosphere, using 5%B4C, 5%KBF4 and 90%SiC powders. Microstructures obtained were characterized by X-ray diffraction, SEM and EDX analyzer. After boronizing, WC and CoW2B2 phases were detected in the both sprayed WC-Co coatings. This suggests that not only WC but also W2C and Co3W3C of WC-Co coatings decomposed by boronizing, resulting in the development of CoW2B2. However, many porosities with a size of more than 10µm were formed on the coating consisting of WC, resulting in a low hardness of HV600. On the contrary, the coating with W2C and Co3W3C has little porosity and a high hardness of HV2600.