An experimental investigation was conducted to study the effect of branch channel diameters on phase distribution behaviors of the two-phase flow in horizontal microimpacting T-junctions. The cross-section of the inlet channel was 1000 mu m x 500 mu m and the cross-sections of the branch channels were 400 mu m x 500 mu m, 600 mu m x 500 mu m, 800 mu m x 500 mu m, respectively. Nitrogen and pure water at 120 kPa (abs) and 25 degrees C were selected as the working fluids. A high speed recording camera was used to clarify the fluid dynamics of the gas-liquid two-phase flow in the test section. Combining above experimental results with the visualization data, the force mechanism of gas-liquid two-phase flow in microimpacting T-junction was analyzed. For the better application of microimpacting T-junctions, the separation efficiency was introduced to evaluate the separation effect. It was found that the uniformity of the phase distribution can be well improved with the decrease of the branch channel diameters. The influencing degree (I) on slug-annular flow was larger than that on slug flow and annular flow. Furthermore, the influencing degree (I) decreased as the superficial velocities of two phases increased when inlet was slug flow or annular flow.