This paper introduces an experimental study on the microbubble generation mechanism in a coflowing microfluidic device. Using propane, butane and air as the dispersed phases and different SDS-PEG solutions as the continuous phases, two kinds of bubble generating phenomena, named "single-bubble generation" flow and "dual-bubble generation" flow, were observed in experiment and uniform bubbles with average diameters ranging from 391 mu m to 713 mu m and polydispersity less than 2.9% were successfully prepared. The pressure fluctuation of gas phase is carefully analyzed with the Young-Laplace equation and the bubble generation mechanism is summarized from the dynamic movement of gas-liquid interface. The main factors impacting the bubble size variation are illustrated for both the organic and inorganic gases. A universal correlation is established to predict the average bubble diameters. (C) 2013 Elsevier Ltd. All rights reserved.