We discuss in this paper a way of analyzing the local characteristics of chemical interaction and utilizing the results of analysis for an under-standing and prediction of chemical reactivities of complicated organic molecules. By applying a paired interacting orbital scheme, the nature of chemical interactions between two molecular species is represented very compactly in terms of the orbitals that are localized on reaction sites or in some specific functional groups of the reagent and reactant molecules. On the basis of these findings, new indices of chemical reactions are presented and are applicable to several types of reactions.