The oily wastewater is generated by various oil and petroleum industries. The discharge of oily wastewater into the natural environment creates a major ecological problem throughout the world. In this work, the effect of application of self-made and commercial nanofiltration membranes was evaluated for treating of oily wastewater. The wastewater was pre-treated by the microfiltration membrane. During the various nanofiltration experiments, the permeated flux was found between 20 and 265 kg m(-2) h(-1). The different applied pressures were 5, 10, 15 and 20 bar, and also different temperatures were 20, 30 and 40 degrees C. The results demonstrated that the chemical oxygen demand (COD) and electrical conductivity (EC) were considerably decreased even more than 10 times lower than initial value in raw and pre-treated feeds. In addition, the permeate flux was increased with increase in applied pressure and temperature. The usage of pre-treatment can also noticeably enhance the efficiency of nanofiltration membrane and wastewater treatment. The response surface analysis was used to achieve the optimum conditions. (C) 2010 Elsevier B.V. All rights reserved.