Micromixers are important devices in microfluidics designed for mixing liquids on a micro scale. These devices are crucial in chemistry, pharmaceuticals, analytical chemistry, biochemistry, and high-throughput synthesis due to their ability to manage small volumes of liquids with precision. In the present work, the mixing process in a converging-diverging micromixer is examined. The micromixer has three mixing units, each consisting of a chamber and an obstacle. For the reference case without an obstacle, molecular diffusion causes mixing at the interface of the two fluids, and with an increase in the number of mixing units and fluid advancement, the effect of molecular diffusion becomes more pronounced. It is also observed that with an increase in input velocity, the mixing index also increases. In the first case, the formation of vortices behind the obstacle leads to greater mixing of the two liquids. Compared to the reference case, the mixing index of the first case increases from 19.12% to 30.21% at an inlet velocity of 0.001 m/s. In the second case, unlike the first one, the maximum mixing index occurs at the lowest inlet velocity. The mixing index for the first and third cases is 34.26% and 56.24%, respectively. However, the pressure drop also increases from 48220 Pa to 58807 Pa. In the fourth case, square obstacles do not significantly increase efficiency, and its efficiency is higher than that of the reference case and lower than that of the other cases.