Using a cluster model Pt21O2, the interaction of O2 with Pt(lll) surface has been studied by means of the atom superposition and electron delocalization-molecular orbital (ASED-MO) theory. From the total energy minimization, we found that the O2 lying-down orientations are favored over the upright orientations, and the chemisorption on bridge site with O-O axis along the bridge is the most stable. The resulting O2 bond length is 1.35?, which is consistent with the latest NEXAFS result of (1.37 ±.05)?. The charge transferred from the substrate to the O2 molecule is about 0.6e, which is mainly filled in the O2 antibonding lπg orbital. An ac-tivation barrier of 0.20eV is found in the dissociation of the π-bonded O2 on the bridge sites. The dissociated O adatoms favor the 3-fold (hcp) hollow sites with a Pt-O bond length of 1.95?. Furthermore, a detailed calculation for local density of states (LDOS) are performed to find the components and origins of UPS features. Finally, a general discussion is given for the O2 phy-sisorption on Pt(lll).