Brown adipose tissue (BAT) has a thermogenic function in animals. Mitochondrial uncoupling protein (UCP) is responsible for the thermogenic function in BAT cells. Vitamin A affects proliferation and differentiation in cells. Retinoic acid, a derivative of vitamin A, stimulates the expression of the UCP gene, which is found in BAT cells. Retinoic acid performs its function regardless of differentiation stage of BAT cells. It is known that vitamin A is teratogenic at therapeutic doses in pregnancy. In this study, effects of retinyl palmitate applied in a therapeutic dose were examined in mice fetal BAT cells. Tissue samples were subjected to a routine electron microscopic procedure for this purpose and then examined. In the control group cells, mitochondria were found to have short cristae, the glycogen granules occupying the cytoplasm were large and the same as those in developing BAT cells, and the lipid granules were few. Glycogen granules constituted smaller clusters in the cytoplasm of cells in the vitamin A group. As compared with the control group, cristae of mitochondria were elongated and matured. In this group, the amount of lipid granules was more striking than that in the control group. In conclusion, this study indicated that vitamin A derivative applied in therapeutic dose during pregnancy accelerates the developmental period of BAT adipocytes in fetal mice.