Underlying malocclusions and dentofacial deformities are often related to variations in the craniofacial development. Type I and type II collagens are considered the major collagens of bone and cartilage respectively. Monitoring the patterns of those protein expressions during development will provide a basis for the understanding of normal and abnormal growths. This study was undertaken to investigate the morphogenetic changes and the expression patterns of type I and II collagen proteins involved in the developing mandible of human embryos and fetuses. 50 embryos and fetuses were studied with Hematoxylin and Eosin, Alcian blue-PAS, Masson Trichrome, and lmmunohistochemical stains. The results were as follows : 1. A 13.5 mm embryo showed the stomatodeum with dental lamina, maxillary and mandibular processes. Meckel´s cartilage appeared in the mandibular arch of a 20.5 mm embryo. New bone formation was bilaterally initiated at the outer side of middle portion of Meckel´s cartilage of 22-38mm embryos. 2. Meckel´ cartilage was resorbed at the 15th week fetus. The endochondral ossification was observed where there was direct replacement of cartilage by bone. Meckel´ cartilage disappeared and membraneous ossification were observed at the 25th week. 3. Before the appearance of Meckel´s cartilage, the expression of type I collagen was moderate at the odontogenic epithelium of maxillary & mandibular process, but mild for the expression of type II collagen. 4. During the appearance of Meckel´s cartilage and new bone formation, the immunoactivity of type II collagen was more expressed than type I collagen at the Meckel´s cartilage and new bone. 5. During intramembranous bone formation, the expression of type II collagen was rare in the bony trabeculae. There was a switch for the expression of collagens from type II to type I during the appearance of Meckel´s cartilage.