In the present study, the formation of air-water plugs at a merging Y-junction was investigated. The generation rates and lengths of gas and liquid plugs were checked for various gas and liquid superficial velocities. For the present experimental range, the generation rates of plugs were increased with increasing gas or liquid superficial velocity, and the effect of the liquid superficial velocity was predominant compared to the gas superficial velocity. The length of gas plugs became longer either by decreasing liquid superficial velocity or by increasing gas superficial velocity. On the other hand, the length of liquid plugs became slightly longer with increasing liquid superficial velocity in the low gas superficial velocity region, and independent of the liquid superficial velocity in the higher gas superficial velocity region. Through the comparison with the previous results of plug formation, the physical interpretations on the present results were provided.