In the present paper, two correlations have been developed to predict the effect of DRP on friction factor of two-phase flow for any pipe diameter. The correlations have been verified using published experimental results of DRP added to air-liquid annular flow and for oil-water flows with any flow pattern at the asymptotic state (maximum drag reduction). Such correlations are not available in literature and considered to be very useful in predicting the drag reduction using DRP and in understanding the most significant parameters that affecting the mechanism of drag reduction by polymers in multiphase flow. (C) 2011 Elsevier Ltd. All rights reserved.