The electronic structure of photo-skinsensitizing psoralens has been investigated by the FMO method. On the basis of theoretical calculations, optimum value of indices (Ft=0.33) has been proposed which corresponds to the sum of frontier electron density. The results indicate that this index is closely correlated with photo-skinsensitizing carcinogenic activity. The formations of molecular complexes between DNA and photo-skinsensitizing carcinogens is discussed in terms of charge transfer interactions.