Pressure drop measurements of air-water slug flows in three circular microchannels (d = 0.486 mm) with various T-junctions (0.136, 0.194 and 0.252 mm) were performed. The measured pressure drops were compared with common predictive models, and it was found that there were deviations between the models and measurements. The tendencies of the deviation between the models and measurements were different for different channels, implying the existence of a T-junction size effect. For confirmation of this effect, a scaling analysis based on the Laplace pressure and Bretherton law was performed. The analysis results showed that the inner pressure of a bubble increased when the T-junction size became small, resulting in drag reduction of the slug flow in the microchannel. Consequently, a modified predictive model for the pressure drop of slug flows was proposed. However, the drag reduction effect decreased when the T-junction was smaller than d/2 because the bubbles cannot maintain their shape without the depressurization due to the geometrical restriction. (C) 2014 Elsevier Ltd. All rights reserved.