Habit planes at WC/Co interfaces were investigated from a viewpoint of carbon content in WC-12mass%Co cemented carbides by transmission electron microscopy (TEM). WC/Co interfaces typically consist of (0001) and habit planes with blunt corners, which are very close to habit plane. The sizes of the blunt corners increase as the carbon content decrease. The expansions of blunt corners are due to the change in interface energy caused by the variation of the carbon content. The change in interface energy is discussed in terms of the incoherency between the (0001) planes of the WC grains and the Co-phase.