The atmospheric oxidation mechanism of 2,7-dimethylnaphthalene (27DMN) initiated by OH radical is investigated at levels of BB1K and G3MP2-RAD//BH&HLYP. The reaction is mainly initiated by OH addition to the C-1 position to form radical adduct RI. In the atmosphere, RI reacts with O-2 via two comparable pathways as direct H abstraction to form 27DMN-1-ol and as O-2 addition to the C-2 position to form RI-2OO radicals, both being slow with rate constants of 10(-18)-10(-17) cm(3) molecule(-1) s(-1). The R1-2OO-s conformer is found to be important in 27DMN oxidation whereas the role of the R1-2OO-a conformer is negligible. Radicals R1-2OO-s have three comparable pathways: ring closure to tricyclic intermediate R1-29OO-s, intramolecular H shift from -OH to -OO to form dicarbonyl products, and reactions with atmospheric NO and/or HO2, etc. The ring closure to R1-29OO-s is endothermic and reversible whereas similar ring closures in benzene and toluene oxidations are exothermic and irreversible. The intramolecular H shift becomes prominent because of the reversibility of ring closure in 27DMN oxidation and is responsible for rapid formation of dicarbonyl compound (C12H12O2) in simulation chamber studies. The oxy radical (R1-2O herein) would not undergo C-1-C-2 cleavage to form dicarbonyl, as suggested in previous studies; instead, R-1-2O would close the ring to form epoxide radical R1-23O. Radical R1-29OO-s would recombine with the atmospheric O-2 and isomerize to diepoxide radical R1-23O-89O-s at comparable rates, and study on their further reactions is desirable.