The effect of atomic-layer deposition (ALD) sidewall passivation on the enhancement of the electrical and optical efficiency of micro-light-emitting diode (µ-LED) is investigated. Various blue light µ-LED devices (from 5 × 5 µm2 to 100 × 100 µm2) with ALD-Al2O3 sidewall passivation were fabricated and exhibited lower leakage and better external quantum efficiency (EQE) comparing to samples without ALD-Al2O3 sidewall treatment. Furthermore, the EQE values of 5 × 5 and 10 × 10 µm2 devices yielded an enhancement of 73.47% and 66.72% after ALD-Al2O3 sidewall treatments process, and the output power also boosted up 69.3% and 69.9%. The Shockley-Read-Hall recombination coefficient can be extracted by EQE data fitting, and the recombination reduction in the ALD samples can be observed. The extracted surface recombination velocities are 551.3 and 1026 cm/s for ALD and no-ALD samples, respectively.