Gas-phase hydrogenation of adiponitrile (ADN) was performed at 1 atm pressure over the Ni-B/SiO2 amorphous catalyst, which exhibited high selectivity to 1,6-hexanediamine (HDA) at 100% ADN conversion. The selectivity to HDA could be further enhanced by MgO dopant. The product analysis revealed that the selectivity to HDA was mainly dependent on the contents of 6-aminohexanenitrile and azacycloheptane formed during the ADN hydrogenation. The 6-aminohexanenitrile was an intermediate in the reaction; it resulted from the half-hydrogenation of ADN; and its content was strongly affected by the activity of the catalyst. The azacycloheptane was a side-product (secondary amine) formed via the nucleophilic addition of the primary amine on one side of ADN chain to the aldimino carbon atom on the other side which was adsorbed by the Ni active sites. The electronic density of the Ni active sites played a key role in determining the content of azacycloheptane. The presence of the alloying B resulted in the electron-enriched Ni active sites which effectively inhibited the formation of azacycloheptane and in turn, resulted in the high selectivity to HDA.