Flow regime transition criteria are of practical importance for two-phase flow analyses at reduced gravity conditions. Here, flow regime transition criteria which take the frictional pressure loss effect into account were studied in detail. Criteria at reduced gravity conditions were developed by extending an existing model from normal gravity to reduced gravity conditions. A comparison of the newly developed flow regime transition criteria model with various experimental datasets taken at microgravity conditions showed satisfactory agreement. Sample computations of the model were performed at various gravity conditions, such as 0.196, 1.62, 3.71 and 9.81 m/s(2) corresponding to micro-gravity and lunar, Martian and Earth surface gravity, respectively. It was found that the effect of gravity on bubbly-slug and slug-annular (churn) transitions in a two-phase flow system was more pronounced at low liquid flow conditions, whereas the gravity effect could be ignored at high mixture volumetric flux conditions. While for the annular flow transitions due to flow reversal and onset of droplet entrainment, higher superficial gas velocity was obtained at higher gravity level. (C) 2011 Elsevier Ltd. All rights reserved.