This paper reports a series of experiments to study the effect of the addition of drag-reducing polymers (DRP) on two-phase flow patterns in a horizontal 0.0254 m pipe. The characteristics of two-phase flow with and without DRP are described. It is noted that the interfacial shear stress decreases sharply by adding DRP and the flow pattern map is changed. Pressure drop reduction occurs in almost all flow pattern configurations. This study indicates that maximum drag reduction usually occurs when a slug, pseudo-slug or annular flow changes to stratified flow by adding DRP. Two flow pattern maps are presented in this paper with and without DRP in the liquid in the pipeline.