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“One-pot regeneration”, which is simple regneneration method of ammonia borane (AB) using hydrazine and liquid ammonia, enables conversion of AB from hexagonal boron nitride (h-BN) after milling hydrogenation. Solution ¹¹B-NMR revealed the presence of AB after NH₃/N₂H₄ treatment of milled h-BN (BNH<i>_x</i>) although the yield of AB was less than 5%. The conversion mechanism was clarified as B-H bonds on the h-BN surface created by ball-milling under hydrogen pressure have an ability to form AB, which was confirmed by Thermogravimetry-Residual Gas Analysis (TG-RGA) and Infrared (IR) analysis. The reaction routes are also the same as regeneration route of polyborazylene because intermediates of AB such as (B(NH₂)₃ and hydrazine borane were found by solution ¹¹B-NMR after soaking BNH<i>_x</i> in liquid NH₃ and hydrazine, respectively. Because of the fact that all reactions proceed on the h-BN surface and no reaction proceeds when neat h-BN is treated, breaking of B₃N₃ ring structure and then creation of B-H bond is the key issue to increase conversion yield of AB.