OBJECTIVE To investigate the effect of bone marrow mesenchymal stem cells (BMSCs) transplantation on the motor function recovery, the expression of vascular endothelial growth factor (VEGF) gene, and angiogenesis after spinal cord injury (SCI) in rats, and to explore the treatment mechanism of BMSCs in SCI. METHODS BMSCs were isolated and cultured from the marrow of 5 Wistar rats (4 weeks old) and the 3rd-4th passage cells were prepared for the experiment. A total of 87 adult female Wistar rats (weighing 220-250 g) were randomly divided into 3 groups: sham-operated group (group A, n=21), DMEM group (group B, n=33), BMSCs group (group C, n=33). A laminectomy was only performed at T8-10 levels in group A. The SCI models were established by modified Nystrom's compression method in groups B and C, and BMSCs and DMEM were injected in groups B and C respectively at 30 minutes after SCI. Basso-Beattie-Bresnahan (BBB) score was used for the motor function recovery at 3, 7, 14, and 28 days, RT-PCR for the VEGF mRNA at 1, 3, and 5 days, and immunohistochemical staining for angiogenesis at 3, 7, 14, and 28 days. RESULTS In groups B and C, the hindlimb locomotor function was improved at different degrees with time, showing significant difference in BBB score between groups B, C and group A (P < 0.05). At 28 days, the BBB score in group C was significantly higher than that in group B (P < 0.05) and there was no significant difference between groups B and C (P > 0.05) at 3, 7, and 14 days after transplantation. The numbers of microvessels in the ventral horns of gray matter around SCI in groups B and C were significantly lower than that in group C (P < 0.05) at 3 days, but there was no significant difference at 7, 14, and 28 days after transplantation (P > 0.05). There was no significant difference in the number of microvessels between group C and group B (P > 0.05) at 3 and 7 days, but the number of microvessels in group C was significantly higher than that in group B (P < 0.05) at 14 and 28 days after transplantation. However, there was no significant difference in the number of microvessels in the white matter around SCI in 3 groups at different time points after transplantation (P > 0.05). The RT-PCR results showed that VEGF mRNA expressed at a low level in group A. Compared with group A, the expression level of VEGF mRNA in groups B and C increased at 1 day and reached the peak at 3 days, then decreased at 5 days after transplantation; and the expression of VEGF mRNA was significantly higher in groups B and C than in group A (P < 0.05), and in group C than in group B (P < 0.05) at 1, 3, and 5 days. CONCLUSION BMSCs may promote the motor function recovery by up-regulating VEGF mRNA expression and increasing angiogenesis in the spinal cord after SCI in rats.