This paper presents operation of a transparent vertical T-junction type separator for flash gas bypass operation with two outlets at the top and bottom for separated vapor and liquid flows. R134a is used as a test refrigerant. Two phase separation load, i.e. inlet flow rate and quality are in the range of 10 g/s to 35 g/s and 10% to 30%, respectively, which are determined primarily according to operation of studied flash gas bypass A/C system. T-junctions with different inlet tube angles of inclination, branch diameter and pre-separation structure are tested to investigate parametric effects on two phase split and liquid separation efficiency in a vertical T-junction. Two phase flow patterns at the inlet of the junction are visualized by high speed camera. Results show that two phase separation process is primarily affected by interaction of gravitational, inertial and vapor phase drag forces and flow regimes at the inlet of T-junctions. Good separation efficiency is achieved when gravitational force dominates over inertial together with drag forces. It is observed that separated or stratified flow pattern at the junction assists phase separation in the vertical branch tube.