Abstract：【Objective】To explore the effect and the possible mechanism of the proteasome inhibitor MG132 on acute T lympho? blastic leukemia cells.【Methods】The influence of different concentrations of MG132 in the viability and proliferation of CCRF-CEM was measured by MTS. Apoptosis rates of CCRF-CEM treated by MG132 were determined by flow cytometry. After being exposed to MG132，the protein levels of FOXO3a in cytoplasm and nucleus were analyzed by Western blotting. qRT-PCR was applied to detect the mRNA of FOXO3a and Puma in cells treated by MG132. Then CCRF-CEM was stably transfected with antisense FOXO3a using Lentivirus infection. We further investigated the effects of MG132 in FOXO3a- shRNA cells and elucidated the mechanisms of FOXO3a and Puma.【Results】MG132 inhibits the proliferation of CCRF-CEM，but has no cytotoxicity in peripheral blood mononu? clear cells（PBMC）. Cellular apoptosis was induced in cells treated with MG132. At mRNA level，MG132 had no influence on FOXO3a，but increased the expression of Puma. However，MG132 promoted the expression of both FOXO3a and Puma at protein level. Interestingly，the expression of FOXO3a increased very little in cytoplasm. In FOXO3a- shRNA cells the expression of FOXO3a and Puma decreased at protein level. FOXO3a’s knockdown attenuated the proliferation inhibition mediated by MG132。 【Conclusion】MG132 inhibits the proliferation and promotes to apoptosis of CCRF-CEM. One of the mechanism is that MG132 inhib?its the degradation of FOXO3a，and then activates FOXO3a/Puma pathway.