Abstract: Objective: To investigate the function of CD4⁺CD25⁺ T regulatory(CD4⁺CD25⁺ Treg) cells in NOD mice.Methods: The frequency of CD4⁺CD25⁺ T cells from pancreatic lympho node(PLN) was analysed,the percentages change over time in the proportion of NOD CD4⁺ CD25⁺ T cells expressing Foxp3 were observed,mean fluorescent intensity(MFI) of Foxp3 in CD4⁺ CD25⁺ FoxP3⁺ T cells was assayed,and in vitro suppression assay was performed.Results: No percentages change over time in the proportion of NOD CD4⁺ CD25⁺ T cells expressing Foxp3 were observed.But the mean fluorescent intensity(MFI) of Foxp3 in CD4⁺CD25⁺FoxP3⁺ T cells decreased with time.In vitro suppression assay,PLN CD4⁺CD25⁺ T cells show decrease in their ability to suppress the proliferation of CD4⁺CD25^- T cells.There exists some relationship between the impairment of in vitro suppression and the reduction at the protein expression levels of Foxp3 on a per-cell basis.Conclusions: It is the impairment of the function of CD4⁺CD25⁺ Treg cells that results in the development of diabetes mellitus in NOD mice.