A micromixer with a three-dimensional serpentine split-and-recombination structure is proposed to provide a high mixing index in a short length, even at low Reynolds numbers. The fluid flow and mixing performance were analyzed numerically using three-dimensional Navier-Stokes equations and the convection-diffusion equation for mass transport in a Reynolds number range of 0.1–200. The mixing performance is represented by the mixing index, which is defined using the mass fraction variance of mixing fluids in a plane. The overall mixing performance of the proposed micromixer is compared with that of previous micromixers. The effects of the number of mixing units on the mixing performance were estimated. A parametric study was performed using five geometric parameters of the micromixer. The proposed micromixer shows mixing indices larger than 0.87 throughout the Reynolds number range in a short length of 1500μm, which is hardly achieved in previous micromixers.