A micromixer is fabricated using a selective high intensity focused ultrasound foaming technique. The micromixer employs a 3D porous region for effective mixing. The 3D porous micromixer can achieve sufficient mixing results with a short mixing length for flows with a Reynolds number as low as 0.1. The fabrication process of the micromixer is rapid, low-cost, and biocompatible. The pore size of the micromixer can be controlled by adjusting the selective high intensity focused ultrasound foaming parameters. The micromixer has potential for use in lab-on-a-chip and micro-total-analysis devices.