The forced-flow convection of water through rectangular microchannels having hydraulic diameters of 0.133-0.367 mm and aspect ratios of H/W = 0.333-1 was investigated experimentally. The flow friction was measured to analyze the heat transfer regimes and to explore the physical aspects of the connective heat transfer. The experimental measurements indicated that the upper bound of the laminar heat transfer regime occurred at a Reynolds number of 200-700, and fully turbulent connective heat transfer was reached at Reynolds numbers of 400-1,500. The transition Reynolds number diminished with the reduction of the microchannel dimension, and the transition range was observed to become smaller in magnitude. For the laminar heat transfer regime, the Nusselt number was found to be proportional to Re^0.62while the turbulent heat transfer case exhibited a typical relationship between Nu and Re, but with a different empirical coefficient, C_H,t. The geometric parameters were found to be important variables that could significantly affect the flow characteristics and heat transfer. The laminar connective heat transfer had a maximum value when the aspect ratio, H/W, was approximately equal to 0.75, and the turbulent heat transfer was optimal when the aspect ratio was in the range 0.5-0.75.