Because of the low Reynolds numbers in microchannels, using of micromixers to improve the flow mixing is essential. Therefore, in this study mixing in nine different micromixer geometries, such as: simple T-shaped micromixer, micromixer with rectangular or parallelogram ribs on the walls of the mixing channel, T-shaped micromixer with two additional parallel or perpendicular inlet channels, micromixer with circular or triangular barriers in the middle of the mixing channel, rhombus micromixer with thick or thin edges, has been investigated. Sinusoidal oscillatory velocity with a phase difference of 180 degrees relative to each other has been applied to channels inlet. The governing equations have been solved numerically using the finite volume method. For all geometries time variation of mixing degree at microchannel outlet and the variation of mixing degree along the channel length have been computed. Results show that for micromixers, which divide the flow to several layers such as rhombus micromixers, mixing degree is high and the micromixers with ribs on the walls have lower mixing degrees. Also, there is an optimum frequency at constant average velocity in which the mixing degree has its highest value.