A horizontally placed Y-junction was used to separate gas and liquid in gas-liquid two-phase flows in a pipe of circular cross section. The original wettability of the Y-junction was good because it was made of acrylic resin. The wettability of one side of the Y-junction was changed to become poor by coating fluororesin. The motions of bubbles and slugs were observed with a high-speed video camera. The bubbles attached preferably to the wall of poor wettability, and accordingly, the separation of gas and liquid was satisfactorily achieved. The efficiency of the separation decreased as the air flow rate increased and it became zero when the gas flow rate exceeded a certain critical value.