Mixing performance has been evaluated for planar split and recombine micromixers with asymmetric sub-channels. The three-dimensional Navier-Stokes equations have been used to analyze fluid flow and mixing in a Reynolds number range from 1 to 80. The widths of the split channels are kept unequal to create unbalanced collisions of the fluid streams. Two shapes of the split channel have been considered: circular and rhombic. In the case of rhombic sub-channels, higher mixing is realized when width of the major sub-channel is either three or four times as wide as the minor sub-channel unlike the case of circular sub-channels. The results show the lowest mixing performance for the case of balanced collisions of fluid streams. The pressure-drop characteristic has been also analyzed at various Reynolds numbers. (C) 2010 Elsevier B.V. All rights reserved.