T-junctions are employed as partial phase separator in the oil and gas industry. The present T-junctions are poorly designed and causing high liquid carry-over with gas phase resulting in malfunction of downstream equipment. This paper presents the novel T-junction design with expander section after the branch arm in order to improve the two-phase separation. The main and run arms of the T-junction are directed along the horizontal plane with the branch arm positioned in the vertical plane. The diameter of the main arm is 0.074 m, with diameter ratio(s) of, 0.67, and 0.33 in relation to branch arm. At the inlet section of the T-junction, the flow regimes generated were stratified, stratified-wavy, and slug flow. At the inlet, the air and water superficial velocities are in the range of 0.25-0.140 m/s and 0.14-0.78 m/s respectively. The effect of the expander section after the branch arm, the air superficial velocity USA and water superficial velocity USW on liquid carry-over (WL3/WL1)max in branch arm have been studied. Based on the experimental data obtained for T-junction with expander section, complete phase separation of air and water was observed in stratified and stratified-wavy flow for all superficial velocities and improved phase separation for slug flow. In slug flow, increasing the liquid superficial velocity improves the phase separation but increasing the superficial gas velocity decreases the phase separation. Finally, phase separation results of new T-junction design is compared with the phase separation data of a simple T-junction. © Universiti Malaysia Pahang, Malaysia