In order to analysis the effect of fetal lung mesenchymal stem cell (FL-MSC) on differentiation of umbilical cord blood mononuclear cells (MNC) into megakaryocytes, the fresh umbilical cord blood MNC were isolated and divided into 2 groups in the culture added with TPO, IL-11 and heparin. In the first group MNC were cultured alone and in the second group MNC were cocultured with FL-MSC. The cells were collected at day 7, 10, 14 for cell counting and detection of CD41a and CD61 by flow cytometry. The morphology and ultrastructure of megakaryocytes were observed by immunohistochemistry method and transmission electron microscopy at day 14. The content of DNA was analyzed by flow cytometry at day 14 too. The results indicated that the of CD41a+ and CD61+ cells were obtained mostly in the second group at day 10 and were in 4.5 and 4.7 fold as much as the MNC cultured alone. The morphology and ultrastructure of megakaryocytes showed immature of nuclei in both of two groups. It is concluded that the FL-MSC could effectively enhance the production of CD41a+ and CD61+ cells, where the effect on nucleus development of the young megakaryocyte was not obviously shown.