Experiments were conducted to investigate the single-phase forced-flow convection of water or methanol flowing through microchannels with rectangular cross-section. The fully developed turbulent convection regime was found to be initiated at about Re = 1000- 1 500. The fully developed turbulent heat transfer can be predicted by the well-known Dittus-Boelter correlation by modifying the empirical constant coefficient from 0.023 to 0.00805. The calculated results are then in quite good agreement with experimental data. The transition and laminar heat transfer behavior in microchannels are very unusual and complex and are strongly affected by liquid temperature, velocity and microchannel size.