Objective：To explore effects of human umbilical cord blood serum on the basic biological characteristics and senescence of human umbilical cord blood mesenchymal stem cells（hUCB-MSCs）. Methods：Mononuclear cells were isolated by density gradient centrifugation from human umbilical cord blood. Two groups of hUCB-MSCs were cultured using different complete mediums，one con－sisting of 3% cord blood and 7% fetal bovine serum（CBS group），the other one consisting of 10% fetal bovine serum（FBS group）. Cell surface moleculars and cell cycle were determined by flow cytometry. Then cell surface area and percentage of positive cells expressing β-galactosidase were detected. Absorbance value was measured by MTT methods，and cell growth curve was drawn. Results：hUCB-MSCs expressed CD44，CD105，CD90，CD73 and vimentin in both groups，but not CD45，CD34，CD11b，CD19 and human leukocyte antigen DR（HLA-DR）. Percentage of CD90 expression in the 7th passage hUCB-MSCs was significantly lower in CBS group than in FBS group. Percentage of cells in G0/G1 phase was significantly lower and proliferation index was significantly higher. Cell surface area was significantly smaller in CBS group than in FBS group（P<0.05）. Growth curve of CBS group was shifted to the left with a higher peak. Cell doubling time was 48.37 h in CBS group，but 54.43 h in FBS group. Percentage of β-galactosidase positive cells was higher in FBS than in CBS group. Conclusions：hUCB-MSCs proliferation is promoted by combining partial human umbilical cord blood serum with fetal bovine serum with better maintenance of basic biological characteristic and delayed cell aging.