Objective：To investigate the effect of irisin on the osteogenic differentiation of rat bone marrow mesenchymal stem cells （BMSCs） under static force and mechanical strain. Methods：Primary rat BMSCs were isolated and cultured，and a four-point bend－ing force system was used to establish an in vitro model of cell stretch. The activity of alkaline phosphatase（ALP） was measured to analyze the effect of irisin concentration on the osteogenic differentiation of BMSCs under static force and mechanical strain and screen out the optimal concentration of irisin. BMSCs were divided into control group，irisin group（100 ng/mL irisin），mechanical strain group（2 000 με mechanical strain），and irisin+mechanical strain group（100 ng/mL irisin+2 000 με mechanical strain）. Cell counting was performed to observe cell proliferation，qRT-PCR and Western blot were used to measure the mRNA and protein expression of osteogenesis-related markers，and the expression of the P38 and ERK 1/2 MAPK pathways was measured. SPSS 20.0 software was used for statistical analysis. Results：The results of ALP activity showed that under static force and mechanical strain，BMSCs treated with 100 ng/mL irisin had a significant increase in ALP activity（P<0.01）. Cell proliferation assay showed that compared with the control group，the irisin+mechanical strain group had a significantly higher level of proliferation of BMSCs（P<0.05）. The results of qRT-PCR showed that irisin + mechanical strain significantly promoted the expression of os－teogenesis-related markers including Runx2，ALP，COL-I，and OPN（P<0.05），and Western blot showed that irisin+mechanical strain significantly promoted the protein expression of Runx2（P<0.05）. In addition，irisin+mechanical strain promoted the proliferation and osteogenic differentiation of BMSCs，which might be associated with the activation of the P38 and ERK 1/2 MAPK pathways. Conclusion：Irisin can promote the proliferation and osteogenic differen－tiation of BMSCs under static force and mechanical strain，possibly via the P38 and ERK1/2 MAPK pathways.