Zero-dimensional (0D) perovskites have recently opened a new frontier in device engineering for light conversion technologies due to their unprecedented high photoluminescence quantum yield as solids. Although many experimental and theoretical efforts have been made to understand their optical behavior, the origin of their green emission is still opaque. Here, we develop a complete experimental and theoretical picture of point defects in Cs-Pb-Br perovskites and demonstrate that bromide vacancies (VBr) in prototype 0D perovskite Cs4PbBr6 have low formation energy and a relevant defect level to contribute to the mid-gap radiative state. Moreover, the state-of-the-art characterizations including atomic-resolution electron imaging not only confirm the purity of 0D-phase of Br-deficient green-emissive Cs4PbBr6 nanocrystals (NCs), but also exclude the presence of CsPbBr3 NCs impurities. Our findings provide robust evidence for defect-induced green luminescence in 0D perovskite nanoscrystals, which helps extend the scope of the utility of these bulk 0D quantum materials in optoelectronic applications.