In this study, saturated flow boiling characteristics of deionized water in parallel microchannels are investigated experimentally. The silicone microchannel heat sink consists of 29 parallel square microchannels having hydraulic diameters of 150 mu m. Experiments have been conducted for four different values of the mass flux consisting of 51, 64.5, 78 and 92.6 kg/m(2)s and heat flux values from 59.3 to 84.1 kW/m(2). Inlet temperature of deionized water is kept at 50 +/- 1 degrees C. Heat transfer and pressure drop are examined for varying values of the governing parameters. Simultaneous high-speed video images have been taken as well as temperature and pressure measurements. The flow visualization results lead to key findings for flow boiling instabilities and underlying physical mechanisms of heat transfer in microchannels. Quasi-periodical rewetting and drying, rapid bubble growth and elongation toward both upstream and downstream of the channels and reverse flow are observed in parallel microchannels. (C) 2016 Elsevier Ltd and IIR. All rights reserved.