In this study, effect of aspect ratio (AR) on the saturated flow boiling characteristics of deionized water in parallel rectangular microchannels are investigated experimentally. Detailed flow visualizations are conducted as well as temperature and pressure measurements. Effects of heat and mass flux are examined, too. Six different silicon microchannel heat sinks, each of which consists of 29 parallel rectangular microchannels, have been used. In order to determine the effect of the aspect ratio (width to depth), the microchannels having different widths and heights but same hydraulic diameters (100 gm) are used. Experiments have been conducted for various values of the mass flux covering 151, 195, 238, 281 and 324 kg/m(2)s, the values of the wall heat flux in the range of 71-131 kW/m(2) and various values of the aspect ratio including 0.37, 0.82, 1.22, 2.71, 3.54 and 5.00. The inlet temperature of deionized water is kept constant at 50 +/- 1 degrees C through the tests. It is obtained that heat transfer coefficient increases with an increase in the aspect ratio up to AR = 3.54 and then decreases. AR = 1.22 has been appeared as a threshold value for the heat transfer coefficient. There is no regular relationship between the aspect ratio and total pressure drop. The flow visualization results provide key findings for underlying physical mechanism. Quasi-periodical rewetting and drying phenomena, rapid bubble growth and elongation towards both upstream and downstream of the channels and reverse flow are observed in parallel microchannels. In addition, strong interactions are obtained between neighboring channels. (C) 2015 Elsevier Ltd. All rights reserved.