This work presents a numerical investigation on mixing and flow structures in microchannels with different geometries: zig-zag; square-wave: and curved. To conduct the investigation, geometric parameters, such as the cross-section of the channel, channel height, axial length of the channel, and number of pitches, are kept constant for all three cases. Analyses of mixing and flow fields have been carried out for a wide range -0.267-267 of the Reynolds number. Mixing in the channels has been analyzed by using Navier-Stokes equations with two working fluids, water and ethanol. The results show that the square-wave microchannel yields the best mixing performance, and the curved and the zig-zag microchannels show nearly the same performance for most Reynolds number. For all three cases, the pressure drop has been calculated for channels with equal streamwise lengths. The curved channel exhibits the smallest pressure drop among the microchannels, while the pressure drops in the square-wave and zig-zag channels are approximately the same. (C) 2009 Elsevier B.V. All rights reserved.