The micromixer that is applied for the electro-osmotic effect in this study, which is a passive type, takes two fluids from different inlets and combines them into a single channel. The fluids then enter the mixing chamber with different inner and outer radii. Four microelectrodes are positioned on the outer wall of the mixing chamber. The electric potentials on the four microelectrodes are sinusoidal for certain values of the time, voltage, zeta potential, and frequency. In order to check the validity of the two-dimensional numerical analysis that already has been performed, we performed a three-dimensional numerical analysis using a commercial code, COMSOL. The incompressible Navier-Stokes equation is solved in this model, with a slip boundary condition on the inner and outer walls of the mixing chamber. The results of two- and three-dimensional models are graphically depicted and compared from the viewpoint of streamlines and concentration.