Our previous results have indicated that Cdc25A is involved in benzo(a)pyrene (BaP)-induced S-phase checkpoint in 16HBE cells and A549 cells. In this paper, we reported the changes of the downstream molecular pathway of Cdc25A and the effects of over-expression and suppression of Cdc25A on BaP-induced S-phase checkpoint. In the S-phase checkpoint induced by BaP the reduction of Cdc25A contributes to cyclin A inhibition. Over-expression of Cdc25A abrogated BaP-induced S-phase arrest in 16HBE cells and concomitantly the expression levels of Cdk2 and cyclin A were not obviously changed by BaP when compared with the control. Cdc25A down-regulation by RNA interference (RNAi) prolonged the S-phase arrest induced by BaP and decreased clearly the expression levels of cyclin A and cyclin E. Therefore, our results further demonstrated that Cdc25A was an effector in Chk1-Cdc25A-cyclin A/Cdk2 pathway of S-phase checkpoint elicited by the carcinogen BaP in 16HBE cells.