Abstract: 2-Ortho, 2-meta and 2-para substituted phenyl-5-biphenyl oxazoles are treated by HMO methods respectively. According to the MO energies and molecular diagrams obtained, substituent effects on the spectro-scopic properties and on the MO index are described with simple molecular orbital theory. Having corrected the parameters under the consideration of ortho effects, a good correlation between the calculation results of ortho compounds and the experimental data of the optical properties is obtained.