BACKGROUND:: MicroRNA plays an important role in the process of growth and aging of living body. To know the role of let-7d in inducing bone marrow mesenchymal stem cell differentiation into neurons can promote the stem cell transplantation. OBJECTIVE:: To investigate the role of let-7d in inducing bone marrow mesenchymal stem cell differentiation into neurons. METHODS:: (1) The lentiviral vector of let-7d was constructed and transfected into rat bone marrow mesenchymal stem cells. The cells were divided into non-transfected group, negative control group (transfected with FU-RNAi-NC-LV), transfected enhancement group (transfected with let-7d-LV), transfected inhibition group (transfected with let-7d-inhibition-LV). (2) Rat bone marrow mesenchymal stem cells were treated with fasudil as an inducer for triggering the cells to differentiate into neurons. The expression of neuron-specific markers, neuron-specific enolase and microtubule-associated protein 2, were measured by immunocytochemical method. The mRNA expression of microtubule-associated protein 2 was detected by RT-PCR. The viability of bone marrow mesenchymal stem cells was determined by MTT method. RESULTS AND CONCLUSION:: Under inverted fluorescence microscope, the cells were successfully transfected with let-7d. Fasudil induced bone marrow mesenchymal stem cells to differentiate into neurons. The transfection efficiency and expression levels of neuron-specific enolase and microtubule-associated protein 2 in transfected enhancement group were higher than those in the negative control group (P < 0.05); while in the inhibition group, they were lower than those in the negative control group (P < 0.05). These findings indicate that let-7d can promote the differentiation of bone marrow mesenchymal stem cells into neurons induced by fasudil, and by controlling the expression of let-7d we can influence the differentiation efficiency from bone marrow mesenchymal stem cells to neurons. SUBJECT HEADINGS:: bone marrow; mesenchymal stem cells; lentivirus; neurons FUNDING:: the National Natural Science Foundation of China, No. 81071114, 81371385Xu XG, Zhang J, Gong Z, Zhao SY, He X, Wang TS, Jiao SJ, Teng JF, Jia YJ. Let-7d lentiviral vector induces the differentiation of rat bone marrow mesenchymal stem cells into neurons in vitro. Zhongguo Zuzhi Gongcheng Yanjiu. 2014;18(41): 6603-6608.