Endohedral metallofullerenes (EMFs) are expected to have a wide range of potential applications in nanomaterials science. Chemical modification of EMFs is very important for their applications. La@C82 possesses 24 non-equivalent carbon atoms. Regioselectivies of reactions of La@C82 is a specially important problem. We applied the Paired Interacting Obitals (PIO) analysis for addition of adamantylidene(Ad), methyl anion(CH₃^-), diethyl bromomalonate anion(DBEM^-) and methyl radical(·CH₃) to La@C82. PIO calculation, proposed by Fujimoto et al., is a method for unequivocally determining the orbitals which should play dominant roles in chemical interactions between two systems, here, La@C82 and each of the above reagents. Eigen values of PIOs, which mean the contribution of PIOs, tell us that the interaction between the reagent and the La@C82 is mainly expressed in only one PIO, PIO-1, in each addition. Considering simultaneously the overlap population(OP) of the PIO-1 and the localization ratio of the PIO-1 of each atom, the most reactive carbons are the carbon(7) in Ad addition, the carbon(23) in DBEM^- addition and carbon(9) or carbon(10) in ·CH₃ addition, respectively,. These results are coincident with the experimental results. A combination of OP and the localization ratio of the PIO-1 is a good reactivity index of each regioselective addition.