Many reports have demonstrated that human bone marrow mesenchymal stem cells (BMMSCs) are resistant to several chemotherapeutic agents or ionic radiation when compared with sensitive tumor cell lines; however, the underlying molecular mechanism is rarely known. In our previous studies, we found that p53 family member p73 was not expressed in BMMSCs with or without the treatment of chemotherapeutic drugs, and the exogenous induction of p73 protein could reduce the resistance of BMMSCs to the drugs. In order to elucidate which factor leads to the inhibition of p73 expression, we used a methylation-specific polymerase chain reaction to investigate the epigenetic methylation status of the p73 gene promoter CpG region. Our data showed that the p73 gene promoter was hypermethylated in BMMSCs but not in tumor cell lines, which were sensitive toward chemotherapeutic agents. Using the demethylation agent 5-aza-2′-deoxycytidine significantly reactivated p73 expression both at the transcriptional and at the protein level. In addition, the treatment of 5-aza-2′-deoxycytidine rendered BMMSCs more sensitive to chemotherapeutic agents through the process of enhanced apoptosis cell death. Taken together, our results suggest that the silencing of the p73 gene mediated by promoter hypermethylation may play a crucial role in leading to the high resistance of BMMSCs to chemotherapeutic drugs and thus we conclude that the p73 gene may be an important element regulating human BMMSCs in response to DNA damage.