The effect of tube diameter on two-phase flow patterns was investigated in circular tubes with inner diameters of 0.6, 1.2, 1.7, 2.6, and 3.4 mm using air and water. The gas and liquid superficial velocity ranges were 0.01-50 and 0.01-3 m/s, respectively. The gas and liquid flow rates were measured and the two-phase flow pattern images were recorded using high-speed CMOS camera. The flow patterns observed were dispersed bubbly, bubbly, slug, slug-annular, wavy-annular, stratified, and annular flows. These flow patterns were not observed in all the test diameters, but were found to be unique to particular tube diameters, confirming the effect of tube diameter on the flow pattern. The data obtained were compared to existing experimental data and flow regime transition maps which show generally reasonable overall agreement at the larger diameters, but significant differences were observed with the smaller diameter tubes.