Here a high-brightness perovskite microcrystalline light-emitting diode (LED) is reported, in which the perovskite microcrystals were grown directly on the conductive substrate and a simple metal-insulator-semiconductor structure was adopted. A peak external quantum efficiency of 0.46% was obtained, which is high for perovskite microcrystalline LEDs. Importantly, the maximum luminance of the device reaches 8848.4 cd m-2, indicating an ultrahigh brightness of >1.2 × 106 cd m-2 for the microcrystals (corresponding to an ultrahigh current density of 80.9 A cm-2), because the light-emitting area of the microcrystals accounts for only ∼0.7% of the device area. In addition, we have studied the degradation of the device at a high current density by in situ microscopic observation and found that a severe Joule heating effect at large injection is the primary problem to be solved to realize electrically pumped perovskite microcrystal lasing.