Flow development and pressure drop were investigated both experimentally and computationally for adiabatic single-phase water flow in a single wide, deep, and long rectangular micro-channel at Reynolds numbers ranging from 196 to 2215. The velocity field was measured using a micro-particle image velocimetry system. A three-dimensional computational model was constructed which provided a detailed description of liquid velocity in both the developing and fully developed regions. At high Reynolds numbers, sharp entrance effects produced pronounced vortices in the inlet region that had a profound influence on flow development downstream. The computational model showed very good predictions of the measured velocity field and pressure drop. These findings prove the conventional Navier-Stokes equation accurately predicts liquid flow in micro-channels, and is therefore a powerful tool for the design and analysis of micro-channel heat sinks intended for electronic cooling.