Abstract: The frictional resistance characteristics of air-deionized water two-phase flow in two vertical rectangular channels with respective cross sections of 1.41 mm×40 mm and 3 mm×40 mm were experimentally studied under atmospheric pressure condition. Eleven classical two-phase flow frictional resistance models were assessed with the acquired 764 data sets. The results show that Lee-Lee model fits well with the data, but there still exists relatively large deviations in high (Re_l>8 700) and low (Re_l<600) liquid Reynolds number regions. In the liquid turbulent region (Re_l≥2 000) the most accurate model is Chisholm B model with the mean absolute errors of 6.13% and 6.43% for the two channels respectively, which however predicts the frictional pressure drop in the liquid laminar region (Re_l<2 000) with large deviations compared to experimental data. The modification to twophase dynamic viscosity is implemented according to the flow resistance characteristics and a modified correlation is proposed for the liquid laminar region, with which the predicted results show a good agreement with the experimental data.