The mechanism of chemiluminescence of 1,2-dioxetane (DO) and 3,3,3,4-tetramethyl-1,2-dioxetane (TMDO) are investigated by the ab initio molecular orbital calculation. The rate-determining step of the chemiluminescent reaction is the O-O bond breaking to form the biradicals. The potential energies along the reaction path are calculated by uB3LYP and uB3P86 methods with 6-31+G(d); the calculated potential barriers are in reasonable agreement with experimental activation energies. An overview of the potential surfaces for overall reaction is obtained. The intersystem crossing mechanism from the singlet biradical to the triplet state is investigated, and the reaction path is followed to the (3)(n pi*) and the reaction path is followed to the (3)(n pi*) excited states of the carbonyl group of respective fragment molecules. The mechanism of promotion to the (1)(n pi*) excited state of formaldehyde is investigated by the MCSCF method on DO. The reaction rates and the yield of chemiluminescence are discussed by the RRKM theory of unimolecular reaction.