A one-dimensional drift-flux model was developed and benchmarked with existing models in the present study. Different from existing empirical correlations, this newly developed 1-D drift-flux model started from the variations of two-phase flow distribution parameter C-0 for non-dimensional superficial gas velocity < j(g)(+)> from low to high, which represents the variation of phase distribution patterns. Due to a combination of small-scale sub-channel effect and large-scale casing effect in a rod bundle, the Hibiki-Ishii's drift velocity V-gj(+) was introduced to describe the relative motion between two phases. A clear boundary of distribution parameter. C-0, can be found when the non-dimensional superficial gas velocity < j(g)(+)> is closed to 0.5. A linearly increasing trend and an exponentially decreasing trend of C-0 were found in the lower and higher gas sides divided by this boundary, respectively. Through the present model, one dimensional area-averaged gas velocity and void fraction can be determined for rod bundles in pool conditions. The prediction results were compared with several existing models and experimental data. The present model shows a good agreement with the existing experimental data and can be applied to both boiling and adiabatic air-water two-phase flows. In order to explain the peak values of C-0 in low superficial gas velocity conditions, several possible superficial velocity and void fraction profiles were examined by considering the recirculation flow/void distributions. (C) 2011 Elsevier Ltd. All rights reserved.