This paper presents a numerical study of the Reynoldsnumber and scaling effects in microchannel flows. The configurationincludes a rectangular, high-aspect ratio microchannel with heatsinks, similar to an experimental setup. Water at ambienttemperature is used as a coolant fluid and the source of heating isintroduced via electronic cartridges in the solids. Two channelheights, measuring 0.3 mm and 1 mm are considered at first. TheReynolds number varies in a range of 500–2200, based on thehydraulic diameter. Simulations are focused on the Reynolds numberand channel height effects on the Nusselt number. It is found thatthe Reynolds number has noticeable influences on the local Nusseltnumber distributions, which are in agreement with other studies. Thenumerical predictions of the dimensionless temperature of the fluidagree fairly well with experimental measurements; however thedimensionless temperature of the solid does exhibit a significantdiscrepancy near the channel exit, similar to those reported byother researchers. The present study demonstrates that there is asignificant scaling effect at small channel height, typically ≤0.3 mm, in agreement with experimental observations. This scalingeffect has been confirmed by three additional simulations beingcarried out at channel heights of 0.24 mm, 0.14 mm and 0.1 mm,respectively. A correlation between the channel height and thenormalized Nusselt number is thus proposed, which agrees well withresults presented.