In the exhaust gas after-treatment, it is difficult to evaluate the oxidation process of diesel soot, because we cannot observe the variation of particle size and the number concentration directly. Moreover, characteristics of soot depend on fuel properties, gas component, and engine conditions. Then, we used carbon particles as model soot, and particle size and its number concentration were experimentally measured in the presence of NO2. The particle size was measured by SMPS, together with the measurement of CO and CO2. Results show that, in the presence of NO2, the oxidation rate is close to the value of diesel soot with catalyzed DPF.