Micro-particle image velocimetry (micro-PIV) is a quantitative full-field technique for the velocity measurement for flows in microfluidics. The flow fields of both Newtonian and non-Newtonian fluids in microchannels were investigated by micro-PIV. The velocity profiles in various polymethyl methacrylate square microchannels were measured. Polyacrylamide aqueous solutions with different concentrations served as non-Newtonian fluids. Two viscous Newtonian fluids were also employed to serve as reference, in particular to validate a lattice Boltzmann (LB) simulation. Satisfactory flow measurements could be realized by the micro-PIV technique. The experimental velocity profile compares well with the LB simulation and an approximate solution. The experimentally measured velocity profile can lead to the quantification of the flow index of non-Newtonian fluids as an inline rheological characterization.