Diverse flow regimes have been encountered in liquid-liquid flows. Some degree of consistency in the observed flow patterns is shown in reported studies, while inconsistency exits when physical properties of the two phases concerned are wide enough. An attempt was made in this study to investigate the mechanisms behind flow patterns of liquid-liquid flows in horizontal pipes. A literature review on flow patterns of liquid-liquid flows in horizontal pipes was conducted. The ratio of the gravitational force to viscous force was proposed to characterize liquid-liquid flows in horizontal pipes into gravitational force dominant, viscous force dominant, and gravitational force and viscous force comparable flow featured with different basic flow regimes. Comparisons of the proposed characterization criterion with the literature data show good agreement. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 1132-1143, 2017