The complete mixing of two or more different fluids in the microscopic scale within a reasonable time period plays an important role in the Micro-Total-Analysis-System (μTAS ). The objective of this study is to propose a new design of a passive micromixer by using flow-field simulations through Computational Fluid Dynamics (CFD) techniques. The primary idea of designing the micromixer is to add feedback side-channels on both sides of the main channel. Through the feedback side-channels, the fluids are guided to flow back into the main channel and to push the main stream and, therefore, the mixing effect will be produced and enhanced. By simulating the flow field in the micromixer, the important design parameters and the operational conditions, such as the channel geometry, the position of side-channel, and the inlet velocity, can be examined.