This work investigates the size-dependent decrease in external quantum efficiency (EQE) of various InGaN/GaN multiple-quantum-well flip-chip blue micro light-emitting diodes (μ-LEDs) of sizes from 10× 10 μm 2 to 250× 250 μm 2 and proposes that the temperature-dependent efficiency droop is the main mechanism for decrease in EQE with reducing dimensions for well-passivated μ-LEDs. Experimental results show that the EQE increases with reducing μ-LED sizes to 50× 50 μm 2. However, the EQE decreases as the μ-LED size is further reduced to 10× 10 μm 2. The measured current-voltage characteristics, the minimum ideality factor, the light-emission patterns by the photon-emission microscope, and the transmission-electron-microscopy images consistently reveal that the decreased EQE of the smallest sized μ-LED is not due to the sidewall leakage: the decreased EQE is rather caused by the temperature-dependent efficiency droop (T-droop), which is systematically found by investigating the blueshift in peak emission wavelength and calculating the thermal resistance (R th) that increases with the reduced mesa area. The decrease in peak EQE at 440 K compared to 300 K is also presented, which demonstrates that the reduction in peak EQE increases with reducing μ-LED sizes. It is pointed out that the small-sized μ-LEDs suffer from higher junction temperature due to lower heat dissipation caused by higher R th compared to large-sized μ-LEDs.