Human umbilical cord blood (HUCB) contains stem/progenitor cells, which can differentiate into a variety of cell types. In this study, we investigated whether HUCB cells differentiate into hepatocytes in vitro and in vivo. We also examined CD34 could be the selection marker of stem cells for hepatocyte. HUCB cells were obtained from normal full-term deliveries, and CD34+/- cells were further separated. For in vitro study, HUCB cells were cultured for 4 weeks, and expressions of liver specific genes were examined. For in vivo study, Non-obese diabetic/severe combined immunodeficient mice were subjected to liver injury by the Fas-ligand carried adenoviral vector or only to be radiated. The mice were treated simultaneously with or without cell transplantation of HUCB, CD34⁺ or CD34^- cells. After 4 weeks, the human specific gene/protein expression was examined. In vitro study, human liver specific genes were positive after 7 days culture. Immunofluorescent study showed positive staining of alpha-fetoprotein, cytokeratin 19 and albumin in round-shaped cells. In vivo study, immunohistochemical analysis showed human albumin, hepatocyte-specific antigen positive cells in mouse liver of Fas-ligand/transplantation group. Fluorescence in situ hybridization analysis using human Y chromosome also showed positive signals. However, no difference between transplanted cell types was detected. In contrast, immunopositive cells were not detected in the irradiated/transplantation group. Reverse transcription-polymerase chain reaction result also showed human hepatocyte specific gene expressions only in the Fas-ligand/transplantation group. HUCB cells differentiated into hepatocyte-like cells in mouse liver, and liver injury was essential during this process. The differences between the CD34⁺ and CD34^- cells were not observed in human hepatocyte specific expression.