We demonstrate an ultrasonication-assisted synthesis without polar solvent of CsPbBr₃ and Cs₄PbBr₆ perovskite nanocrystals (PNCs) and their reversible transformation. The as-prepared CsPbBr₃ PNCs and Cs₄PbBr₆ PNCs exhibit different optical properties that depend on their morphology, size, and structure. The photoluminescence (PL) emission and quantum yield (QY) of the CsPbBr₃ PNCs can be tuned by changing the ultrasound power, radiation time, and the height of the vibrating spear. The optimized CsPbBr₃ PNCs show a good stability and high PL QY of up to 85%. In addition, the phase transformation between CsPbBr₃ PNCs and Cs₄PbBr₆ PNCs can be obtained through varying the amount of oleylamine (OAm) and water. The mechanism of this transformation between the CsPbBr₃ PNCs and Cs₄PbBr₆ PNCs and their morphology change are studied, involving ions equilibrium, anisotropic growth kinetics, and CsBr-stripping process.