The single-phase forced convective heat transfer and Row characteristics of water in microchannel structures/plates with small rectangular channels having hydraulic diameters of 0.133-0.367 mm and distinct geometric configurations were investigated experimentally. The results indicated that the geometric configuration had a significant effect on the single-phase convective heat transfer and flow characteristics. The laminar heat transfer was found to be dependent upon the aspect ratio and the ratio of the hydraulic diameter to the center-to-center distance of the microchannels. The turbulent heat transfer was found to be a further function of a new dimensionless variable, Z, such that Z = 0.5 will be the optimum configuration for turbulent heat transfer regardless of the groove aspect ratio. The friction factor or flow resistance reached a minimum value as Z approaches 0.5. The turbulent flow resistance was usually smaller than that predicted by classical relationships, and the Reynolds number for flow transition to fully developed turbulent flow became much smaller than the ordinary channel flow. Empirical correlations were suggested For calculating both the heat transfer and pressure drop.