Environmental carcinogen benzo(a)pyrene (BaP) generates electrophilic products BaP diolepoxide (BPDE) that react covalently with genomic DNA. Cells that acquire BaP/BPDE-induced DNA damage undergo S-phase arrest in a p53-independent manner. However, the role of Cdc25A in the BaP/BPDE-induced checkpoint is not clear. In the present study, we investigated the change of checkpoint kinase 1 (Chk1) and Cdc25A in S-phase arrest elicited by BaP. The results indicated that BaP (10microM, with S9 mixture) treatment induced S-phase arrest in both human lung carcinoma A549 cells and human bronchial epithelial cells line 16HBE cells, increasing the proportions of cells in S-phase 19.0% and 21.1%, respectively, at 12h after treatment, compared with DMSO control (p<0.01). Then, the S-phase arrest was weakened after 24h. The level of phorsphorylated Chk1 obviously increased and Cdc25A protein level decreased in both two cell lines after treatment with BaP. The results of RT-PCR indicate Cdc25A mRNA in both A549 cells and 16HBE cells was not changed after BaP treatment 12h, and 24h. The treatment of the proteasome inhibitor MG132 greatly increased Cdc25A protein in abundance. Over all, our results indicated Chk1-Cdc25A checkpoint pathway is involved in BaP-induced S-phase arrest. Moreover, transcription of Cdc25A did not change in BaP induced S-phase arrest, the decrease of Cdc25A level was due to increased degradation through the ubiqutin-proteasome pathway.