Transcription factors P53 and FOXO are both activated in response to stresses via protein–protein interactions, leading to events such as cell survival or apoptosis. To clarify the mechanisms that regulate FOXO activity, we analyzed the intermolecular interaction of FOXO3a and P53. FOXO3a and P53 interacted in COS-7 cells, and transcriptional activity of FOXO3a was suppressed by P53, but P53 was not affected by FOXO3a. RT–PCR revealed that expression of the endogenous apoptosis-inducible genes Bim and Bcl6 was decreased markedly by co-expression of P53 with them, but expression of p27 and CyclinG2 was not. In addition, treatment with 500 μM H2O2 for 30 min to 1 h to mimic oxidative stress promoted protein binding. Serum deprivation and drug treatment also affected the binding of FOXO3a and P53. These findings suggest that FOXO3a controls cellular function by changing its molecular interaction with P53 to mediate transcription factor activity under stress stimuli.