The aim of this study was to detect DNA damage during expansion ex vivo of umbilical cord blood (UCB) hematopoietic cells and explore the optimal harvest time for culture of CB hematopoietic cells. Mononuclear cells (MNCs) separated from UCB were cultured in a serum-free system supplemented with cytokines and colony forming units were assessed by semisolid culture at the same time. On day 0, 7, 14 and 21 cells were collected for single cell gel electrophoresis (SCGE) analysis and CFUs were also assayed by SCGE, CD34+ cells and CD133+ cells were quantitated by fluorescence-activated cell sorting (FACS). The results showed that the percentage of CD34+ and CD133+ cells was found to be highest after short-term culture (<14 days) and the cord blood DNA damage rate was observed to be less than 5.0% at earlier time points, but at day 21 the DNA damage rate was 28.2%, which was higher than that at day 0 (p=0.000), the tail length of the DNA comet was longer than that at day 0 (p=0.000). The tail lengths of DNA damage on other time points were not significantly different from that at day 0. It is concluded that the DNA damage rate is less than 5.0% after short-term (<14 days) culture of UCB cells ex vivo by using this method. After 14 days DNA damage rate increases significantly. The optimal harvest time of cord blood cells after culture ex vivo would be within 14 days.