The purpose of this study was to clarify the mechanism of articular cavity formation by means of observing the temporomandibular joint (TMJ) in human fetuses (9th-30th gestational weeks) using light microscope. In the 9th-11th week, although neither cavity had been formed, several small blood vessels running postero-anteriorly on the lower surface of the articular disk that would be the future lower cavity were recognized. However, such blood vessels were not seen in the future upper cavity beneath the glenoid fossa. In the 12th week, when the lower cavity formation commenced, the cavity was filled with blood corpuscles, and a series of apertures was observed throughout the condyle, from the posterior portion to a part of the anterior portion. Tracing posteriorly, the lower cavity diminished gradually until it had at last united to one blood vessel. On the other hand, in upper cavity formation, which was recognized only at the posterior portion of the TMJ, no blood corpuscles were seen in the upper cavity but several small clusters of collagen fibers. Blood corpuscles in the lower cavity disappeared after the formation of synovial membrane (20th week) and vascular canals in the condyle (21st week). From these findings, the mechanism of lower cavity formation appears to differ from that of upper cavity formation. Lower cavity formation involved small blood vessels running postero-anteriorly on the lower surface of the articular disk. As for upper cavitation, there was no evidence of blood vessels, which would suggest that upper cavitation depends on another mechanism of formation.