Experiments are conducted with a perfluorinated dielectric fluid, Fluorinert FC-77, to investigate the effects of channel size and mass flux (225-1420 kg/mzs) on microchannel flow boiling regimes by means of high-speed photography. Seven different silicon test pieces with parallel microchannels of widths ranging from 100 to 5850 mu m, all with a depth of 400 mu m, are considered. Flow visualizations are performed with a high-speed digital video camera while local measurements of the heat transfer coefficient are simultaneously obtained. The visualizations and the heat transfer data show that flow regimes in the microchannels of width 400 mu m and larger are similar, with nucleate boiling being dominant in these channels over a wide range of heat flux. In contrast, flow regimes in the smaller microchannels are different and bubble nucleation at the walls is suppressed at a relatively low heat flux for these sizes. Two types of flow regime maps are developed and the effects of channel width on the flow regime transitions are discussed. (C) 2009 Elsevier Ltd. All rights reserved.