Systematic investigation of the electronic properties of fullerenes C-N (N =60, 70, 76, 78, 82, 84, 86) has been made by cyclic voltammetry, optical absorption, and photo-ionization measurements, The HOMO-LUMO gap decreases monotonically with increase of number of carbon atoms (N) and shows a characteristic 1/N-dependence comparing with those of polyenes and aromatic hydrocarbons. As a reflection of large molecular size, the Coulomb-repulsive energy, U’, is considerably small less than 0.4 eV and decreases further with increase of molecular size.