Effects of applied DC electric field on flow boiling heat transfer in a vertical minichannel heat sink for two mass velocities and different heat fluxes are investigated experimentally. R141b is employed as working fluid. The heat sink contains eight rectangular minichannels, as well as both width and height of a single minichannel are 2 mm. Wire electrode is used and placed in the center of minichannel to create a nonuniform electric field. Flow visualization is performed using a high-speed video camera to analyze the interfacial features with and without electric field. Furthermore, the mechanism of two-phase flow boiling heat transfer enhancement under an electric field is explored. The results show that the onset of nucleation boiling slightly shifts towards downstream under electric field conditions. Electric field brings more vapor production, leading to early transition from bubbly flow to slug flow and early transition from slug flow to churn or annular flow. Electric field causes the bubbles to oscillate between the wall and the wire electrode, or renders the bubbles intermittently to touch the heated wall in the bubbly flow region. In the slug flow region, the surface of the elongated bubble intermittently touches the heated wall, and the interfacial shape irregularly changes with the bubble growing under electric field conditions. Overall, applying electric field can enhance the flow boiling heat transfer performance in the minichannel heat sink, and the enhancement effect is more significant in the bubbly and slug flow region. Moreover, the maximum enhancement ratio of 2.48 is obtained under present experimental conditions. (C) 2018 Elsevier Ltd. All rights reserved.