Micromixers have important applications in various pharmaceutical and medical fields. In the present study, the enhancement of mixing index in electroosmotic micromixer with different geometries is investigated. The commercial software COMSOL Multiphysics 5.4 is employed to solve the mathematical models. The SIMPLEC algorithm is employed for coupling the velocity and pressure fields. A second order upwind scheme is used to reduce the artificial diffusivity. The results show a remarkable effect of the electric field on the mixing efficiency. The optimum geometry is the one with no obstacle in the mixing chamber. For the optimum geometry, it is demonstrated that the mixing efficiency increases with the voltage, however there are optimum values for frequency and inlet velocity in which the micromixer exhibits its best performance. The optimum values of frequency and inlet velocity are 8 Hz and 0.1 mm/s, respectively. It is revealed that the micromixer with no obstacle can reach the mixing efficiency of about 97%.