In this paper, we have shown that the coefficients of LCAO MO of molecules with periodical structures can be splitted into two factors, one indicating the positions and the other the physical properties of the atoms. The first factor is palled the geometrical factor, and the second the physical factor. In terms of these factors, the quantum chemical calculations of molecular orbitals and energy levels are greatly simplified. We have calculated the LCAO MO and energy levels of coronene and polycene; and have given the simplifid seonlar equations for polyphenyl and peri-condensed diphenyl.