Platelet-reinforced WC-Co alloys are processed by liquid-phase sintering from very fine-grained WC powders in the presence of small amounts of TiC. Large and flat WC grains develop in the material. The microstructure of these platelets is investigated by high-resolution electron microscopy in order to obtain information on their formation mechanism. Inside the grains, an extended defect parallel to the basal plane is observed. It can be described by a pair of stacking faults with a shear vector equal to 1/3 < 0-110 > occurring in two successive (0001) planes. At the level of the faults, the plane spacing is slightly reduced. The defect area is similar to a thin cubic layer about 0.5 nm thick at the interior of the platelet. The enhanced grain growth of the platelets is likely related to the presence of the defect area.