Objective To investigate the effect of human bone marrow mesenchymal stem cells (BM-MSCs) stimulated by platelets in vitro on the metastasis of cancer cells.Methods The BM-MSCs were isolated and cultured in vitro and platelets from the peripheral blood of healthy persons were purified.The MSCs (control),platelets + MSCs,and platelets treated with culture media (CM) of SGC-7901 tumor cells + MSCs (T-platelets + MSCs) were cultured,respectively,and the MSCs and supernatants (MSCs-CM and SGC-7901-CM) were collected,respectively,after 24 hours.The expressions of markers of cancer-associated fibroblasts (CAF),such as α-SMA and Vimentin,were determined by Western-blotting.The immigration ability of BM-MSCs were analyzed by Transwell assay.The levels of P-selectin in platelets stimulated by MSCs-CM or SGC-7901-CM were detected with flow cytometry.The metastasis model of gastric cancer SGC-7901 cells was established in BALB/c nude mice by the injection of tail vein,and the tumor metastasis in vivo was observed.Results The expression levels of P-selectin in platelets stimulated by MSCs-CM ([21.37 ± 1.00] ％) or SGC-7901-CM ([31.4 ± 1.71] ％ were significantly higher than that in the control ([3.17 ± 0.40] ％,t =27.85 and 29.18,P ＜ 0.01).The expression levels of α-SMA and Vimentin in platelets + MSCs group (0.79 ± 0.08 and 0.88 ± 0.01) and T-platelets + MSCs group (0.90 ±0.06 and 0.96 ±0.04) were significantly higher than that in the control (0.64 ±0.02 and 0.75 ±0.05,t =2.96 and 6.45 forα-SMA,t =4.73 and 5.73 for Vimentin,P ＜0.01).The amounts of immigration cells in platelets + MSCs group (340.3 ±27.7) and T-platelets ± MSCs group (424.3 ± 17.6) were significantly higher than that in the control (220.7 ± 19.4,t =6.14 and 13.48,P ＜ 0.01).The in vivo experimental results showed that the metastatic foci in platelets ± MSCs group (4 ± 2) and T-platelets ± MSCs group (21 ± 4) were significantly higher than that in the control (0.33 ± 0.06,t =3.051 and 8.857,P ＜ 0.01).Conclusion Platelets promote the immigration and the enhanced tumor metastasis in vivo of BM-MSCs.