An experimental study has been carried out to explore the laminar hydrodynamic development length in the entrance region of adiabatic square microchannels. Flow field measurements are acquired through the use of micro-Particle Image Velocimetry (micro-PIV), a non-intrusive particle tracking and flow observation technique. With the application of micro-PIV, entrance length flow field data is obtained for two different microchannel hydraulic diameters of 500 μm and 100 μ, both of which have cross-sectional aspect ratios of one. The working fluid is distilled water, and velocity profile data is acquired over a laminar Reynolds number range from 0 to 200. The test sections were designed as to provide a sharpedged microchannel inlet from an infinitely sized reservoir, at least 100 times wider and higher than the microchannel hydraulic diameter. Also, all microchannels have a length-to-diameter ratio of at least 100, to assure fully developed flow at the channel exit. The micro-PIV procedure is validated in the fully developed region with comparison to Navier-Stokes momentum equations. Good agreement was found with comparison to conventional entrance length correlations for ducts, and no influence of scaling was observed.