In real-time quantitative PCR, the accuracy of normalized data is highly dependent on the stability of the reference genes. However, reference gene expression in a given cell type or experimental condition can vary considerably. The goal of this study was to establish a reliable set of reference genes for real-time PCR studies using human umbilical cord mesenchymal stem cells with long-term in vitro expansion. The stability of ten potential reference genes was examined in human umbilical cord mesenchymal stem cells. We found that Ywhaz and Rpl13a, not beta-actin or Gapdh, were the most stably expressed of the internal control genes in different passages of human umbilical cord mesenchymal stem cells. Ywhaz and Rpl13a could be used as reference genes for relative gene quantification and normalization purposes in real-time PCR studies of human umbilical cord mesenchymal stem cells.