According to the increasing distance between the processing facilities and the oil and gas production sites, especially at subsea production sites (where multiphase flows commonly occur), innovative methods are demanded to help reducing the cost of multiphase flow transportation through the pipelines. In the past decades, Drag Reducing Polymers (DRPs) have been drawing attention in industry and academia. However, in these works it was predominately focused on single phase flows while limited attention was paid to multiphase flows. In order to improve the understanding of the influences of the DRPs on horizontal stratified gas-liquid flows, a conventional air-water flow loop is used in this study. The “Conductance probes method” and flow visualization with high speed camera are used to examine the morphology of the gas-liquid interface. The conductance probe sensors and required electrical circuits are built as a part of this project. The stratified flow of gas and liquid provides the most suitable flow configuration to validate the equipment which was built to measure the time variations of the liquid film height. The effects of drag reducing polymers on stratified gas-liquid flows and particularly the effects of DRPs on the interfacial phenomena as the crucial characteristics of the stratified flows are discussed in this study. Polyacrylamide with the molecular weight of 15000 kg/mol is used in this study. To get more insight on the rheological properties of polymeric solutions, the characteristics of solutions including static and dynamic surface tensions, dynamic and complex viscosities are measured. In order to find a link between these properties and the results obtained from the flow loop tests, the effects of polymer concentration on these properties are also investigated. In this research the relevance of the gas and liquid flow rates to the drag reduction phenomenon is studied. The highest drag reduction observed in this study (about 55 percent) is obtained at the greatest liquid and gas superficial velocities. The maximum drag reduction of a stratified flow is compared with the one obtained from single phase flow of water. Additionally, in this study it is tried to focus on the behavior of the interface between the gas and the liquid by adding polymers. Roll waves’ frequency, velocity and shape as well as the influences of the drag reducing polymers on the roll wave’s properties are discussed as well. Finally a model is proposed to assess the drag reduction of stratified flows as a function of the changes in liquid holdup and the interfacial properties such as disturbances and the shape of the interface.