Abstract A passive micromixer featuring convergent–divergent channel walls with sinusoidal variations is proposed in this work. The main channel splits into two sub-channels that recombine at regular intervals along the microchannel’s length. To evaluate mixing performance, steady Navier–Stokes equations with a diffusion–convection model for species concentration are solved for Reynolds numbers in the range of 10 ⩽ Re ⩽ 70. A mixing index based on the variance of the mass fraction of the mixture is employed to evaluate the mixing performance of the micromixer. The effects of the Reynolds number, the amplitude of the sinusoidal walls, and the aspect ratio of the channel on mixing performance are investigated. The results indicate that a symmetric double vortex pair appeared at the throat of the convergent–divergent channel. And, secondary motions as a result of differential centrifugal forces originating in the sub-channels and throat of the convergent-divergent walls significantly affect mixing behavior. The proposed micromixer shows significantly improved mixing performance compared to a similar geometry based on the concept of unbalanced split and collisions for a wide range of Reynolds numbers. Also, the pressure drop is acceptable for a variety of lab-on-a-chip and micro-total analysis systems.