A new set of experimental pressure drop air-water flow data at microgravity conditions obtained aboard the NASA KC-135 aircraft is reported. Comparisons between pressure drop values at mu-g and l-g vertical upward how suggest that the forced convection two-phase flow frictional pressure drop at microgravity is of the same order of : magnitude as that at normal gravity for otherwise the same tube geometry and flow conditions. The main reason seems to be that the flow is mainly inertia dominated over the range of liquid and gas flow rates tested. The experimental data were compared with several widely used empirical models, e.g. homogeneous model Lockhart-Martinelli method and Friedel’s model. All models gave reasonable predictions.