Slug flow behavior and characteristics have been studied in a rectangular microchannel with Y-junction. Two simple bubble shape models are proposed to calculate the film thickness, gas hold-up and the specific surface area. The results show a significant effect of inertia on the bubble generation process. The liquid film thickness was greatly thickened by the inertial effect. The main forces on the rupture of the gas bubbles present a shifting from pressure (squeezing regime) to shearing stress and dynamical pressure of the liquid (shearing regime). The transition was found to mainly depend on the liquid velocity. A strong leakage flow of the liquid film around the gas bubbles was suggested as the liquid slug lengths decreased instead when liquid velocity increased. This leakage flow is also the reason of the over-prediction of the bubble velocity based on the stagnant film hypothesis. (c) 2013 Elsevier Ltd. All rights reserved.