Quantitative determination of platinum oxidation state on supported platinum catalyst was examined by using Pt LIII-edge XANES spectra. The nano-sized platinum particles on the support were partially oxidized under oxidative atmosphere. The atomic ratio of oxygen to platinum (O/Pt) for these particles could be determined by the amount of desorbed oxygen in He carrier (O2-TPD) during elevated temperature. On the other hand, the XANES spectrum of the platinum catalysts oxidized under each corresponding condition was recorded under the same condition, then the area intensity of the white line of these spectra was evaluated by a simple simulation using a set of Gaussian and arctangent functions. A good linear relationship was confirmed between the white line intensity and the atomic ratio of oxygen to platinum, meaning that the oxidation state of the platinum could be quantitatively determined by a simple XANES analysis.