The dehydrogenation of pristine and defective h-BN sheets-supported ammonia borane (AB) systems is investigated by the first-principles method. The results show that nitrogen or boron atoms at the edges of vacancies are used as hydrogen acceptors which plays a crucial role in the process of dehydrogenation between the defective h-BN sheets and AB. The energy barriers to the transfer of hydrogen from AB to the defective h-BN sheets are low and the processes are significantly exothermic, indicating that these reactions are thermodynamically favorable. The hydrogenated defective h-BN sheets (protonic and hydridic hydrogens transfer from AB to the defective h-BN sheets) further react with AB to release H-2 with low reaction barriers. Our theoretical study demonstrates that the defective h-BN sheets can be the effective metal-free catalysts. Furthermore, the potential advantage of such dehydrogenation process is the fact that no other elements are being introduced into the system. (C) 2015 Elsevier B.V. All rights reserved.