The morphogenesis of Valsalva's sinus of semilunar valves in chick embryo was studied with SEM and LM. The sinus began to develop at stage 30 and was basically formed at stage 34 (based on H-H staging). The endothelial cells on arterial and ventricular surfaces of the valve were morphologically different. Mesenchymal cells beneath the endothelium were distributed loosely or densely on different parts of the valve in accordance with the morphological changes of the endothelial cells. The study suggests that the formation of the sinus is associated with not only the growth of endothelial cells on the arterial surface of the valve into underneath mesenchyme, but also with the mesenchyme which is loosing and presents some cavities.