PAHs are one of the major pollutants in aquatic environment. The Coastal marine area having such type of pollutant frequently added by the halting of fishing vessels, oil leakages, drainages from various sources. Perna viridis is the sturdy green mussel normally found to live in estuaries and costal marine beds which are the portal entry for land based pollutants. In order to overcome the pollution risk the animals defense come for rescue thus the elevated levels of different types of enzymes as a biomarker whether its elevation rate of hepatic cytochrome P450-associated enzyme activity. The inducibility and activity of phase I reduction nicotinamide adenine dinucleotide phosphate, reduced (NADPH) cytochrome c reductase (CCR), cytochrome c oxidase (COX) and three CYP450 isoforms (benzyloxyresorufin - O-dealkylase [BROD], ethoxyresorufin-O-dealkylase [EROD] and methoxyresorufin-O-dealkylase [MROD] ) enzymes were measured in the hepatic S9 fraction prepared from Perna viridis collected from three sites: a highly oil-polluted site (Kasimedu fishing harbor, Rayapuram, Chennai [Station1]); a moderately polluted off-shore site, about 3 Km away from this area [Station 2]; and the least oil-polluted site (Vellar estuary, Parangipettai [Station 3], which was a reference site) and also PAHs treated with the same animal at different concentrations in the laboratory. The inducibility and activity of phase I reduction nicotinamide adenine dinucleotide phosphate, reduced (NADPH) cytochrome c reductase (CCR), cytochrome c oxidase (COX) and three CYP450 isoforms (benzyloxyresorufin - O-dealkylase [BROD], ethoxyresorufin-O-dealkylase [EROD] and methoxyresorufin-O-dealkylase [MROD] ) enzymes were measured in the hepatic S9 fraction prepared from Perna viridis collected from the above sites and also treated with ethyl-naphthalene at different concentrations in the laboratory condition within 24hr duration. The levels of BROD (CYP2B6), MROD (CYP1A2) and EROD (CYP1A1) in the tissues of green mussel were measured using Spectro-Fluorometer. All the MFO enzymes exhibited a hierarchical dose-dependent activity in response to oil pollution in these study areas. Samples from the heavily oil-polluted (Kasimedu Station-1 and Station-2) areas exhibited greater activity of all enzymes than the least oil-polluted (Vellar estuary) reference area. In the laboratory treated hepatic tissue also elevated the MROD. Among the enzymes analyzed, the MROD activity was best correlated with the level of hydrocarbon contamination. Therefore MROD can be considered as a robust biomarker for petroleum hydrocarbons in P. viridis.