Abstract: AIM:To observe the effects of osteoprotegerin (OPG) as an initiating factor on the changes in the number of endothelial cells and on nitric oxide synthase (eNOS) and nitric oxide (NO). METHODS: Immortalized human endothelial cells were conventionally cultured and then treated with different concentrations of OPG (0, 0.5, 1.0, 2.0, 4.0, 6.0, 8.0, 10.0 and 100.0 ng/ml, total nine groups) for 48 h. Number of cells was detected with the cell count kit (CCK-8). Nitric oxide production was detected with nitric oxide detection kit, eNOS protein expressions was examined with immunocytochemical method and the level of eNOS mRNA was assayed with realtime quantitative PCR. RESULTS: Compared with the number (0.759±0.067) in control group, the number of endothelial cells gradually increased with the increase of OPG within the physiological concentration range. The increased number of cells in 8.0 ng/ml group and higher concentration groups were statistically significant, (1.091±0.200) in 8.0 ng/ml group (P<0.05), (1.103±0.152) in 10.0 ng/ml group (P<0.01) and (1.231±0.192) in 100.0 ng/ml group (P<0.01). NO increased synchronously with the proliferation of endothelial cells and also reached statistical significance in 8.0 ng/ml and higher groups (102.224±0.612) in 8.0 ng/ml group (P<0.05), (117.183 ± 0.552) in 10.0 ng/ml group (P<0.01) and (128216±0.492) in 100.0 ng/ml group (P<0.01). eNOS protein and mRNA expression in 6.0 ng/ml, 8.0 ng/ml, 100 ng/ml and 1000 ng/ml groups increased significantly in a concentration dependent manner (P<0.01). CONCLUSION: OPG within the human physiological concentration range promotes endothelial cell proliferation. During the process of antiatherosclerosis, OPG may promote endothelial cell repair and the effect is positively correlated with the OPG concentration gradient.