Abstract: The domestic and international research situation and developing direction of the oil-water two-phase flow patterns in horizontal pipeline have been summarized. The flow patterns have been divided into stratified flow and dispersed flow. A further subdividing is as followed: smooth stratified flow, wavy stratified flow, wavy stratified flow with liquid droplet layer, oil droplet dispersion layerwater stratified flow, water droplet dispersion layer-oil stratified flow, water-in-oil dispersed flow and oil-in-water dispersed flow. The major factors affected the flow patterns and pressure drop have been analyzed, including pipe wall materials, entrance shapes, and physical properties of oil and so on. The conversed conditions of flow pattern from stratified flow to semi-dispersed flow have been determined by analyzing interface stability. And the conversed conditions of flow pattern to fully dispersed flow have been determined by comparing the maximum droplet size with the critical droplet size. Also, the stratified pipe-flow theoretical solution and the conventional mathematical models of stratified flow in bipolar coordinates are introduced such as double-layer fluid model, the improved three-layer flow model and four-layer fluid model.