Twinned WC grains are sometimes observed in WC powder and sintered WC-Co alloys. The present investigation has studied the formation of twinned WC grains during carburization of an Eta phase. Eta grains were carburized at 700-1450°C for 1 min to 9 h. Twinned WC grains formed during the carburization. Crystallographic characterization of the formed twins were made using SEM and TEM. The formation of twins was found to be affected by the carbon activity during carburization. The twins formed under high carbon activities while no twins formed under low carbon activities. Two kinds of twins with different orientations were observed. The present experimental observation suggests that the twins formed via 2-dimensional nucleation and layer-bylayer growth on small WC clusters under high supersaturation and high driving force for the growth of WC grains.