Based on the viscous Kelvin-Helmholtz theory, an instability analysis of stratified flow during condensation inside horizontal tubes is conducted to predict the transition from stable two-phase stratified flow to unstable flow. Taking into account the condensation heat transfer, a linear analysis of the one-dimensional two-fluid model is employed to calculate the amplification factor, which is used to quantify the instability of a liquid-vapor two-phase stratified flow. It emphasizes the response of the amplitude of the disturbance wave at the liquid-vapor interface to condensation heat transfer. The results show that mass transfer originating from condensation heat transfer is apt to increase this amplitude and ultimately accelerates the transition from stable stratified flow to an unstable transitionalflow regime bounded between stratified and slug flow. In a liquid-vapor two-phase flow regime map the phenomenon is depicted as a much narrower stable stratified region than in the case of the adiabatic flow. Meanwhile, the stable flow region will shrink rapidly with increasing the saturation-to-wall temperature difference. All of results are found to be in agreement with the available experimental data very well. @ 2015 Elsevier Ltd. All rights reserved.