This study investigates the heat transfer characteristics and flow pattern for the dielectric fluid HFE-7100 within multiport microchannel heat sinks with hydraulic diameters of 480 m and 790 m. The test results indicate that the heat transfer coefficient for the smaller channel is generally higher than that of the larger channel. It is found that the heat transfer coefficients are roughly independent of heat flux and vapor quality for a modest mass flux ranging from 200 to 400 kg m-2 s-1 at a channel size of 480 m and there is a noticeable increase of heat transfer coefficient with heat flux for hydraulic diameters of 790 m. The difference arises from flow pattern. However, for a smaller mass flux of 100 kg m-2 s-1, the presence of flow reversal at an elevated heat flux for hydraulic diameters of 480 m led to an appreciable drop of heat transfer coefficient. For a larger channel size of 790 m, though the flow reversal is not observed at a larger heat flux, some local early partial dryout still occurs to offset the heat flux contribution and results in an unconceivable influence of heat flux. The measured heat transfer coefficients for hydraulic diameters of 790 m are well predicted by the Cooper correlation. However, the Cooper correlation considerably underpredicts the test data by 35-85% for hydraulic diameters of 480 m. The influence of mass flux on the heat transfer coefficient is quite small for both channels.