All-inorganic mixed-halide CsPb(Br1-x I x )3 perovskite nanocrystals (NCs) are excellent candidates for green-emitting phosphors in wide color gamut displays; however, a detailed investigation of their photoluminescence (PL) properties based on the halide composition has been missing. In this work, we report a fundamental investigation of the changes in the PL properties of CsPb(Br1-x I x )3 NCs. The PL color of the NCs, which were prepared by a hot-injection method, changed from green to red with increasing iodide composition (x). Almost ideal green emission close to the chromaticity coordinates of the green vertex of the BT.2020 standard was achieved by appropriately substituting iodide ions for bromide ions in monohalide CsPbBr3 NCs. However, the PL peak width of the mix-halide NCs at x ∼ 0.5 was 0.127 eV, which was broader than the 0.105 eV peak width of the monohalide CsPbBr3 NCs. This phenomenon should be due to the compositional inhomogeneity among the individual CsPb(Br1-x I x )3 NCs. On the other hand, the PL quantum yield (PLQY) for the monohalide CsPbBr3 NCs decreased from 70 to 25% as x increased to 0.5. This result may be attributed to lattice distortion by the difference in the ionic radii of bromide and iodide. Improvements in the compositional inhomogeneity and lattice distortion would enhance the color purity of the green emission and the PLQY, respectively, of the CsPb(Br1-x I x )3 NCs.