In this study, real petrochemical wastewater treatment was investigated at four hydraulic retention times (HRTs) of 30, 18, 12 and 6 h, using membrane bioreactor. Initial evaluation confirmed the presence of styrene and more than 30 different types of compounds in petrochemical wastewater. Laboratory tests and gas chromatography-mass spectrometry (GC-MS) studies indicated that at all HRTs, styrene and chemical oxygen demand (COD) removal efficiencies are higher than 99 and 95%, respectively. Further investigations also revealed that HRT decreasing, not only does not reduce the COD removal efficiency, but also improves the effluent COD by increasing mixed liquor suspended solids. Also GC-MS analysis indicated that styrene as the major pollutant was removed over 99%. Transmembrane pressure was monitored for determining membrane fouling. Data showed severe membrane fouling occurred at HRT of 6 h which was attributed to increase in soluble microbial products.