The reactivity of aminoboranes has been studied theoretically. We have derived the unoccupied reactive orbitals localized on the boron p pi atomic orbital and the occupied reactive orbitals localized on the nitrogen lone-pair orbital in these compounds. The Lewis acidity of the boron center and the Lewis basicity of the nitrogen center have been estimated, by which the effect of substituents on the reactivity of aminoboranes is systematically examined. The results show that the substituent effects in aminoboranes are somewhat different from those in other boranes and amines owing to the p pi-p pi conjugation. The experimentally observed reactivity trend in [2 + 2] cycloaddition reactions has been found to be understood in terms of local characteristics of aminoboranes as a Lewis acid and as a Lewis base. This view has been suggested to be applicable to other types of reactions of aminoboranes.