Abstract: ［Abstract］Objective To understand the effect of regulatory T cells on senile osteoporosis by measuring the percentage of regulatory T cells in CD4+T cells in different age groups of male SD rats. Methods Sprague-Dawley male rats which was 6-8 weeks，6 months，12 months，24 months old were selected as subjects. There were 10 rats in each group. We detected bone mineral density of femur，spine，pelvis，whole-body of rats by dual-energy X -ray absorptiometry. We collected blood samples from the heart，used CD25/forkhead box P3 double-positive to describe regulatory T cells and then measured the percentage of regulatory T cells in CD4+T cells by flow cytometry. Results（1）The percentage of regulatory T cells in CD4+T cells in 6-8 weeks，6 months，12 months，24 months old male rats groups were not the same， the difference was statistically significant（P＝0.000）. The percentage of regulatory T cells in CD4+T cells of 6-8weeks old groups was lower than 6 months，12 months，24months old groups， the difference was statistically significant（P＝0.000）. The percentage of regulatory T cells in CD4+T cells of 6 months old groups was lower than 12 months，24 months old groups， the difference was statistically significant（P＜0.05）. The percentage of regulatory T cells in CD4+T cells of 24 months old groups was lower than 12months， but there was no significant difference（P＞0.05）.（2）There was a positively correlation between the percentage of regulatory T cells in CD4+T cells and bone mineral density of femur，spine，pelvis，whole-body. Conclusions （1）The percentage of regulatory T cells in CD4+T cells in four groups are not the same，the lowest at 6-8 weeks of age，by 6 months，there is an increase and when 12 months increase to a maximum then decrease at 24 months of age，indicating that the percentage of regulatory T cells in CD4+T cells in different age groups are significantly different. （2）There is a positive correlation between the percentage of regulatory T cells in CD4+T cells and bone mineral density of femur，spine，pelvis，whole-body，indicating that because of the bone resorption reducing with the percentage of regulatory T cells increasing，the values of bone mineral density raising，when the percentage of regulatory T cells decreasing，the values of bone mineral density declining. In view of the number of regulatory T cells declining with aging，suggesting that exogenous injection of regulatory T cells may inhibit the bone resorption by osteoclast，which may become the new direction of senile osteoporosis treatment.