Due to easy manufacturing technology and implementation of passive micromixers in a complex microfluidic system, in this study, a type of passive micromixer has been investigated. Passive micromixers increase the mixing rate by increasing the contact surface of two fluids and reducing the distance of molecular diffusion. In the present study, numerical analysis of an L-micromixer has been performed to investigate the mixing behavior and characteristics of fluid flow with changes in key geometrical parameters at four Reynolds numbers. Three non-dimensional geometrical parameters, i.e., normalized length (ZR), length ratio (LR), and aspect ratio (AR) have been defined. Simulations have been performed at the Schmidt number of 900.18. The Reynolds number has been also selected in the range of 50 to 200. A mixing index has been used to quantify mixing behavior in the microchannel. The accuracy of simulation done has been proved by comparing current results with the results of other valid studies. The results reveal that mixing index and pressure drop in a serpentine channel are sensitive to the changes of geometric parameters of the microchannel, and showing different behavior at various Reynolds numbers. Furthermore, due to the sharp 90° turns in the microchannel, the inertial force is large enough to cause vortices, which leads to chaotic advection.