Objective: To investigate the mechanism of AICD in CD4 CD25 regulatory T cells (T). Methods: Murine CD4 CD25 T cells from naive BALB/c or DO11.10 mice were isolated by MACS CD4 CD25 T cells from BALB/c mice were stimulated with anti-CD3/CD28 antibodies, and those from DO11.10 mice were incubated with OVA peptide plus APC respectively. Real time PCR was applied to detect the gene expression pattern in CD4 CD25 T cells. Flow cytometry was performed to determine the apoptosis of CD4 CD25 T cells. The influence of anti-FasL, anti-TRAIL neutralizing antibody and zVAD-fmk on AICD of CD4 CD25 T cells was evaluated. Results: CD4 CD25 T cells were classified by MACS with 98% purity and specifically express the Foxp3 gene which is the 'master control gene' for T cells and could suppress the proliferation of CD4 CD25 T cells. The apoptosis of freshly isolated CD4 CD25 T cells was less than 5%, but could reach to 39%-45% after being activated in vitro for 8 days. The results of real time PCR showed different gene expression pattern of TNF family member in freshly isolated CD4 CD25 T cells and activated CD4 CD25 T cells. The apoptosis of CD4 CD25 T cells which induced by anti-CD3/CD28 antibody or OVA peptide plus APC was significantly inhibited by either anti-FasL neutralizing antibody, or anti-TRAIL neutralizing antibody and caspase inhibitor zVAD-fmk as well. Conclusion: FasL/Fas and other apoptosis-related molecules might be involved in the AICD of CD4 CD25 T cells.