Experimental results have been obtained for single-phase forced convection in deep rectangular microchannels. The microchannels were fabricated in a 2 mm thick silicon substrate by means of chemical etching. The tested configuration has 251 μm wide channels and 119 μm thick channel walls. The channel depth is 1030 μm and the channels cover a total projected area of 2.5 cm by 2.5 cm. A thin-film heater is deposited on the back side of the silicon substrate, corresponding to the entire projected channel area. The silicon substrate measures 2.9 cm by 2.9 cm, with only a 2 mm wide edge surrounding the channel area. All tests were performed with deionized water as the working fluid, where the liquid flow rate ranged from 5.47 cc/s to 118 cc/s. A critical Reynolds number of 1500 was found for this configuration, contrary to that of larger channels. The theoretical analysis as well as previous data found in the literature agree reasonable well with the experimental findings. This channel configuration has been shown to reduce temperature non-uniformity in the substrate compared to previous studies by utilizing relatively high flow rates. In addition, the theoretical analysis shows that increasing the channel depth can significantly improve the flow and heat transfer performance.