Abstract: Green mussels (Perna viridis), exposed to 0 μg · L^-1, 5 μg · L^-1, 10 μg · L^-1, 50 μg · L^-1 and 100 μg · L^-1 No.0 diesel oil water-soluble fraction (WSF) for 15 d under semi-hydrostatic simulation, were transferred to clean sea water for 7 d to analyze the gene expression levels of CYP4 by the method of 2^-△△Ct combining with real-time quantitative PCR. Results show that WSF obviously induced CYP4 up-regulation and was differentially expressed in both visceral mass and mantle in P.viridis. The expression level of CYP4 was obviously higher in visceral mass than in mantle. The relative gene expression level initially increased then decreased and increased again as time elapsed in those two kinds of tissues. The most significant difference was observed in 50 μg · L^-1concentration group (P < 0.01)compared to the control. At the last 7 d, the expression level in visceral mass and mantle decreased quickly and that in some groups gradually returned to normal level by being transferred to clean sea water.