As a promising material for optoelectronic devices, cesium lead halide perovskite nanocrystals have attracted wide attention recently. However, the fast anion exchange among CsPbX₃ is a problem for applications (e.g., light emitting diode, LED) that require different colors. In this work, we report the large-scale synthesis of ultrathin cesium lead bromide (CsPbBr₃) nanoplates (NPs) of tunable edge length and thickness via a simple one-pot colloidal approach. The thickness of NPs can be precisely tuned in a monolayer level by varying the reaction kinetics. The high-quality ultrathin NPs can be prepared in large scale of 0.25 L/batch. The ultrathin CsPbBr₃ NPs emit blue light due to the strong quantum confinement effect, in contrast to the green emission of bulk CsPbBr₃. For the first time, a blue LED device has been successfully fabricated by using ultrathin CsPbBr₃ NPs as the active layer. The use of CsPbBr₃ perovskite as the emitting layer for blue LED may promote the development of next-generation LEDs and displays.