A micromixer is a component of a lab-on-a-chip or microfluidic device that mixes two or more chemicals together. The purpose of this study is to assess the performance of passive micromixer of various shapes. Six shapes of micromixers compared mixing performance. Three dimensional modeling was carried out to have the same hydraulic diameter. The commercial code, ANSYS Fluent, was used to simulate the internal mixing flow. A numerical analysis method is described in detail in this paper. The performance of the micromixer was compared with the mixing index and pressure drop. Consequently, the CDM-8T shape showed reasonable mixing performance and relatively low pressure drop.