The origin of WC/WC grain boundaries in liquid-phase-sintered WC-Co alloys has been investigated in a WC-Mo2C-Co model system using coarse WC polygrain powder. The evolution of grain shape during liquid phase sintering was able to be identified by observing a growth layer that contained Mo. During liquid phase sintering, most of the grain boundaries in the powder were penetrated by a Co liquid but some of them were not. Electron backscattered diffraction analysis confirmed that some boundaries in the powder, in particular, Sigma 2 twist boundaries and Sigma 97 special boundaries, remained intact during liquid phase sintering. These experimental results confirm that the grain boundaries of WC grains in liquid-phase-sintered WC-Co alloys originated from those present in the starting powder.