Abstract The semi-empirical ASMO SCF method has been applied to the peptide molecule. In this treatment, σ and π electrons are explicitly taken into account. With regard to σ electrons, the molecular orbital originally used by Sandorfy (1955) and extensively developed by Fukui et al . (1962) in the Huckel MO method is employed, and the integrals involved in this treatment are estimated semi-empirically. The calculated absorption maximum, its oscillator strength, ionization potential and dipole moment are compared with experimental data and good agreement is found. The important role of the hydrogen atom in semiconductivity of protein and the possibility of σ-σ ∗ , π-σ ∗ and σ-π ∗ transitions of the peptide molecule are pointed out.