Aim: Our objective in this study was to investigate ultrastructure of endothelial and muscle cells of human umbilical vessels.Material and Methods: Ten umbilical cords were used for this study. These cords represented periods of gestation from sixteen weeks to full-term. The specimens were fixed in 5 % buffered glutaraldehyde for four hours at 4oC. The sections were postfixed for two hours in a solution of 1% osmium tetroxide prepared with Millonig phosphate buffer. Sections were dehydrated in ethanol and propylene oxide and then embedded in araldite. They were stained with uranyl acetate, examined electron microscopically.Results: Rod- shaped bodies were observed in the arterial endothelium in the early stage as described by Weibel-Palade. Large numbers of mitochondria and increased lipid and vacuoli structures and increased Weibel-Palade bodies were also noted in full-term. the muscle cells of the umbilical vein were characterized by prominent crystalized structures of mitochondria at sixteen weeks.These structures were degenerated at full-term, and myelin figures were observed.Conclusion: The endothelial and muscle cells of the umbilical vessels from early stage to full-term showed morphological changes.