Abstract: Based on the VOF (Volume of Fluid) method, a three-dimensional unsteady mathematical model is developed to study the gas-liquid two-phase flow in horizontal pipe under different gravities of 10^-4g₀, 0.17g₀, 0.38g₀ and 1g₀ (g₀=9.8m·s^-2). The flow patterns and their transitions are numerically investigated by the developed model and the void fraction distribution and oscillation are presented. The results indicate that the characteristics, such as flow patterns, void fraction, and slip ratio, of gas-liquid two-phase flow under different gravities can be well predicted by the developed model. Under the same two-phase superficial velocity, the gas phase accumulates and coalescences in the upper part of horizontal pipe with increasing gravity, resulting in the transition of flow patterns. In addition, with increasing gravity, the slip ratio increases while the peak and frequency of section void fraction oscillation decrease. However, due to that the increasing two-phase superficial velocity enhances the inertia force of two-phase flow, the gravity effects are weakened.