An effective planar micromixer based on a meandering microchannel with converging-diverging cross section is proposed, fabricated and investigated in this work. The side walls of the microchannel consist of successive semi-elliptical surfaces with perpendicular major axes. The results of this work show that the fluid mixing in such a microchannel is enhanced by the multidirectional vortices, including the Dean vortices in the transverse plane due to centrifugal force in the meander-channel flow and the separation vortices caused by the converging-diverging cross section of the meandering channel for the case with a large flow rate and a small enough radius of curvature. We fabricate the micromixer by a single lithography process and the fluid mixing in the micromixer is observed by using a confocal spectral microscope imaging system. The fluid mixing is also simulated by a commercial code. The simulation results are in reasonable agreement with the experimental results. The results show that the mixing efficiency of the present micromixer with a large expansion ratio defined as the maximum width divided by the minimum width of the main channel is generally better than that of the ordinary meander-channel micromixer with constant cross section for the case with a large flow rate and a small radius of curvature. Besides, more simulation results show that the present meandering-channel micromixer formed by semi-elliptical side walls with perpendicular major axes is superior to the meandering-channel micromixer formed by semi-elliptical side walls with aligned major axes, the meandering-S micromixer and the Sigma micromixer. (C) 2012 Elsevier B.V. All rights reserved.