Objective: Observe the modification BCL-2, BAX,Caspase3,Caspase6 and Caspase9 about their gene and protein expression and the activity adjustment of Caspase3,Caspase6 and Caspase9 in rat hippocampus, aimed to explore the mechanism of apoptosis induced by Benzo(a)pyerne.Methods: Fifty healthy SD rat were distributed into five groups characterized as blank control group, olive control group, 1.0, 2.5 and 6.25mg/Kg BW B[a]P group.Rats were intra-peritoneal injected with B[a]P of different dose separately every other day for 90 days.Then Morris water maze was used to measure learning and memory ability of them.Use flow cytometry to test apoptosis ratio of neuron in rat hippocampus.Detect the expression of apoptosis correlated mRNA and protein by using fluorescent quantitation PCR and western blot.And test the activity of apoptosis correlated protein using spectro-photometry.Results: Average escape latency of 2.5and 6.25 mg/Kg B[a]P groups were significantly increased and Frequency of cross platform decreased compared to blank, Olive control group and 1.0 mg/Kg B[a]P group, still platform quadrant detention and platform quadrant detention ratio of 6.25 mg/Kg B[a]P group was significantly decreased (p<0.05);Results came from flow cytometry showed that early apoptosis ratio were increased as the dose of B[a]P ascend(ptrend<0.05) and early apoptosis ratio of 1.0, 2.5 and 6.25 mg/Kg B[a]P group were significantly higher than blank and Olive control group.BAX protein was up-regulated at 6.25 mg/Kg B[a]P group while BCL-2 protein and BCL-2/BAX down-regulated.Caspase3,6 protein was significantly rised at 2.5 and 6.25 mg/Kg B[a]P group compared to blank, Olive control group and 1.0 mg/Kg B[a]P group (p<0.05), but we didnt observed modification of Caspase9 protein (p>0.05).Activity of Caspase3,Caspase6 and Caspase9 were increased at 2.5 and 6.25 mg/Kg B[a]P group, and significantly more than the tow control groups and 1.0 mg/Kg B[a]P group(p<0.05).And there exist a positive correlation between Caspase3, Caspase6, Caspase9 activity and early apoptosis ratio of neuron in rat hippocampus(r=0.793,p=0.019;r=0.886,p=0.006;r=0.773,p=0.025).Expression of apoptosis correlated gene in hippocampus present no significantly modification (p>0.05).Conclusion: Subchronic B[a]P expose can induce apoptosis of neuron in hippocampus;Overexpress of BAX, caspase3,caspase6 and up-regulate of caspase3,caspase6 and caspase9 activity, coupled with deregulate of BCL-2and BCL-2/BAX would be one of the apoptosis mechanism induced by B[a]R