The onset of nucleate boiling in the flow of water through a microchannel heat sink was investigated. The micro-channels considered were 275 mu m wide by 636 mu m deep. Onset of nucleate boiling was identified with a high-speed imaging system and the heat flux at incipience was measured under various flow conditions. An analytical model was developed to predict the incipient heat flux as well as the bubble size at the onset of boiling. The closed-form solution obtained sheds light on the impact of the important system parameters on the incipient heat flux. The model predictions yield good agreement with the experimental data. (c) 2005 Elsevier Ltd. All rights reserved.