Possible geometrical structures and relative stabilities of ［Mg(NH2)2］n(n=1—5) clusters are studied by using the hybrid density functional theory (B3LYP) with 6-31G* basis sets. For the most stable isomers, the electronic structure, vibrational properties, bond properties and ionization potentials are analyzed. The calculated results show the following tendencies: the Mg and N atom are bonded with each other to form catenulate structures. The bond lengths for ［Mg(NH2)2］n (n=1—5) clusters are about 0.190—0.234nm for Mg—N, and 0.101—0.103 nm for the N—H bonds, the bond angles of H—N—H are about 100.2°—107.5°. The population analysis suggests that the natural charge of N atoms are about -1.551e—-1.651e, that of Mg atoms are about 1.585e—1.615e, that of H atoms are about 0.369e—0.403e, and that of—NH2 are about -0.784e—-0.845e, and the bonds between Mg and —NH2 have strong ionicity. The comparative study of structures and spectra of clusters and crystal show that —NH2 keeps the integrity in the crystal and in clusters.