We investigate the oxidation mechanism of carbon nanotube caps through first-principles pseudopotential calculations. An oxygen molecule is found to adsorb
on the cap with the energy barrier of about 2 eV. We find that the bond breaking of the O-O and C-C bonds at the adsorption sites can occur with small energy barriers, forming more stable geometries. This process is dominant for the 6-6 bond than for the 6-5 bond at the cap. We also consider adsorptions on the perfect carbon nanotube wall and find that the cap oxidation is preferred than the wall oxidation, which is consistent with experimental results.