An experimental study on flow patterns for water boiling in horizontal heated tubes was conducted to observe the evolution and transition of How patterns while controlling various parameters, such as heat flux, mass velocity, and tube diameter. Six boiling flow patterns are observed in the test tubes, namely, bubble, plug, slug, wave, stratified, and annular flows, which can be further classified into three types based on their various characteristics: intermittent, stratified-wave, and annular flows. The evolution of flow patterns for water boiling in horizontal heated tubes was significantly affected by heat flux, mass velocity, and tube diameter. The increment of heat flux pushed the starting point of each flow pattern toward the inlet of the tube. Intermittent and stratified-wave flows occurred at lower mass velocity, and annular How occurred with an increase in mass velocity. Moreover, annular flow easily appeared in tubes with smaller diameters. Since the heat per unit of mass for the full tube length increases with either an increase in heat flux or a decrease in mass velocity and tube diameter, the transition always occurs at smaller vapor qualities with the increase of the heat per unit of mass during the transition of the three flow patterns. The experimental data in this study were compared with the adiabatic How pattern maps proposed by Baker, Mandhane, Taitel, and Weisman. Based on the experimental data, a new flow pattern map for water boiling was proposed for predicting flow patterns in horizontal heated tubes. (C) 2013 Elsevier Ltd. All rights reserved.