In the present study, effect of channel diameter and liquid viscosity on formation of gas and liquid plugs at a merging Y-junction was experimentally investigated. Two Y-junctions were tested, having 1.58 and 2.41 ㎜ inner diameters, respectively. Air was used as the gas phase whereas pure water and aqueous solution of glycerol were used as the liquid phases. As the channel diameter of Y-junction was increased, the generation rates of gas and liquid plugs formed at merging Y-junction were decreased. However, the liquid viscosity has little influence on the plug generation rate for the present experimental condition.