AbstractA novel active micromixer design involving an oscillating diaphragm is proposed. A simulation study is performed to determine the effect of diaphragm length, amplitude, and frequency of the oscillating diaphragm on the mixing index. The geometry is a simple T‐joint with an elongated channel for the outlet with a diaphragm on the outlet channel wall. The fluids at the inlets are similar in fluid properties with the same flow rate and diffusion constant. The mixing index saturates with increasing frequency and amplitude of oscillation for a given diaphragm length. The diaphragm's effectiveness was calculated based on the pressure difference between the diaphragm inlet and outlet zone. The effectiveness declined with an increase in the frequency of the diaphragm oscillation. An analytical expression is derived to find the effect of the frequency and amplitude of the diaphragm oscillation on the mixing index. This simple novel design may find wide applications in various industrial fields.