Gene expression of P450 in Juveniles of Acipenser persicus in exposure to sub lethal doses of cadmium chloride (200, 400 and 800 µg/l) were studied during 14 days. Sampling was done from the both liver and gill on days 1, 2, 7 and 14. Result showed that in gill and liver tissue, the relative mRNA- P450 expression level significantly increased in all experiment days compared to control(P<0.05). Relative mRNA- P450 expression level showed a clear time- dependent response in both tissues after exposure to CdCl2. The amounts of mRNA expression were increased in the highest dose in all studied days in this investigation. The same trends were observed in all studied doses (200, 400 and 800 µg/l), in both examined tissues. Although an increase in relative mRNA- P450 expression was observed in both tissues in this study, its relative expression was more in liver than gill tissue (p<0.05). In addition, the relative mRNA- P450 expression was higher in higher concentrations.