This study was aimed at the effects of mechanical stretching on DNA synthesis of human periodontal ligament fibroblasts(PDLF). Flow cytometry(FCM) was used to examine the proliferative activities of PDLF grown on the flexible culture dishes subjected to mechanical stretching. The results showed that in the range of 0.5%-8%, mechanical stretching could stimulate DNA synthesis 2 hours after the cells had been subjected to stretches. DNA synthesis reached the peak in the 6 hours. The effect of 2% stretching was more significant. As time went by, the mitogenic effect of stretch was decreased. These findings suggest that mechanical stretching can stimulate DNA synthesis of PDLF.