A new facile, cost-effective and highly efficient cyclization of sodium aminodiboranate induced by I₂ is developed. The method can be used to prepare aminodiborane and its N–methyl/N, N–dimethyl derivatives, which is catalyst-free, performance independent of temperature and N–substituted groups. Theoretical calculations demonstrate that a two-step cyclization mechanism is favored in the cyclization of sodium aminodiboranate induced by I₂, where the intermolecular hydrogen bond and the (η²-H₂) dihydrogen moiety play an important role, and the iodination of boron does not occur.