Recently, the structural stability of all-inorganic halide perovskite nanocrystals has been significantly enhanced. To understand the enhancement, we developed surface-passivation models for cubic CsPbBr3 nanocrystals with anionic (oleate) and cationic (oleylammonium) organic ligands based on first-principles calculations and nuclear magnetic resonance investigations. We propose that the (100) surface is initially terminated with oleate ligand complexes on PbBr2(100) surfaces. Also, the ligand transition to oleylammonium on the Pb-rich surfaces is expected due to the addition of metal halides (ZnBr2) during colloidal synthesis. The significant improvement in the structural stability of the cationic ligand-passivated CsPbBr3 nanocrystals was attributed to the suppressed exposure of the merging-vulnerable (110) surface, caused by the large difference in formation energy between the ligand-passivated (100) and Br-passivated (110) surfaces.