Confined Impinging Jet Reactors (CIJR) have become an important new reactor type used in the chemical industry and have been successfully applied in chemical plants. However, there have been few investigations on asymmetric CIJRs. In this work, the micromixing efficiency of an asymmetric T-shaped CIJR consisting of two tubes with inner diameters of 3 mm and 6 mm was investigated using a new parallel competing iodide iodate reaction. The effects of operating conditions such as impinging velocity and reagent concentration on the mixing performance were studied. The experimental results show that the segregation index X-S decreases with an increase in impinging velocity and impinging jets momentum ratio, and the decrease of acid concentration. Further, the distance between the axes of the jets and the top wall of the mixing chamber has a very complicated effect on the micromixing efficiency in the asymmetric CIJR. Based on the incorporation model and the experimental data, the micromixing time was estimated to be of the magnitude of milliseconds, confirming that the asymmetric CIJR possesses a superior micromixing efficiency.