The photooxygenation reaction of large polycyclic aromatic hydrocarbons (PAHs) composed of more than six benzene rings was investigated by using the semiempirical molecular orbital method. The optimization of the PAHs and their endo-peroxides revealed that oxygen attaches to a pair of carbon atoms which have the largest pi-electron density of the highest occupied molecular orbital. The structure of the endo-peroxide of tetrabenzo[de,hi,op,st]pentacene, determined experimentally, could be reproduced by the calculation. The heats of formation for the PAHs, endo-peroxides, and transition states were also calculated, and an energy correlation diagram of the reaction was obtained. The experimental results for the rate of photooxygenation were explained in terms of the energy difference between the initial and transition states in the profile.