In this study the effect of the presence of a drag reducing agent (DRA) on the pressure drop in cocurrent horizontal pipes carrying slug two-phase flow of air and crude oil is investigated. An experimental set-up is erected. The test section of the experimental set-up is consisted of. a smooth pipe of polycarbonate with 10.3 m long and 2.54 cm ID, a rough pipe of galvanized iron with 8.8 In long and 2.54 cm ID and a rough pipe of galvanized iron with 8.8 m long and 1.27 cm ID. The employing DRA is a Polyalpha-olefin (Polyisobutylene). The percent drag reduction (%DR) is calculated using the obtained experimental data, in presence of the DRA. The results show that addition of DRA Could be effective up to some doses of DRA after which the pressure drop is kept constant. A %DR of about 40 is obtained for some experimental conditions. (c) 2005 Elsevier Ltd. All rights reserved.