Objective To observe the ultrastructure of human periodontal ligament cells after the osteoinductive culture. Methods Human periodontal ligament cells(hPDLCs) were cultured in the osteoinductive medium with 10SUP>-7/SUP> mol/L dexamethasone, 50 mg/L α-scorbic acid and 10SUP>-2/SUP> mol/L β-glycerolphosphate or in ordinary culture for 14 days.Transmission electron microscopy was used to observe the ultrastructural changes. Results After osteoinductive culture,there were richer mitochondria,endoplasmic reticulum and Golgi complex in the cytoplasm of hPDLCs than that in the ordinary clutured hPDLCs.And lots of myelin like structures and matrix vesicles with middle or low electron density were observed in the hPDLCs cultured with the osteoinductive medium.Conclusion The ultrastructural characteristics of hPDLCs after the osteoinductive culture were similar to those of mineral-forming cells,such as osteoblast and c