Subcooled flow boiling in microchannels plays a significant role in solving heat dissipation issues in many industrial applications. Nevertheless, the fundamentals of flow boiling and bubble dynamics in confined microchannels are lack of sufficient and clear comprehension, especially considering the effect of the geometric parameters. The present work aims at revealing the critical structural parameter that determines the bubble behaviors and heat transfer performance in the different microchannels. By high speed photography, the features of bubble transversal coalescence and the variations of liquid film thickness in the different microchannels are captured and compared, which strongly proves the theoretical discussion about the effect of the geometric parameters. The experiment results indicate that the bubble transversal growth is linearly dependent on R-E(3/2), and determined by the geometric parameter w(c)(2)h(c), while the aspect ratio (AR) plays the decisive role in bubble elongation mode and rate. Meanwhile, with a series of crossover experiments, the influence of the inlet flow rate and heat load on the subcooled flow boiling in confined microchannels are also clearly revealed. By analyzing the heat transfer capability caused by these structural differences, some insights might be provided for the better application of confined microchannels. (C) 2017 Elsevier Ltd. All rights reserved.