The introduction of small amount of drag reducing polymer (DRP) to horizontal oil-water flow is known to influence flow pattern and pressure drop along the flowline. In this study the efficiency of DRP on two pipes made from acrylic material with diameter 25.4 and 19.0mm was investigated. Mineral oil and co-polymer (Magnafloc 1035) were used. Polymer concentrations between 2 to 30 ppm were investigated. The results showed that pipe diameter has significant effect on drag reduction. Drag reduction of up to 60% was observed in the 25.4 mm pipe while only 50% was achieved in the 19.0 mm pipes. Stratified flow extended to higher superficial oil velocities in the smaller pipe diameter in comparison with larger one. Annular flow was found to disappear from both pipe after polymer addition. Finally, variation of master solution concentration did not affect the drag reduction.