A new method was introduced to detect the concentration of OH radical in dielectric barrier discharge (DBD) reaction. A film, which was impregnated with salicylic acid, was used to detect OH radical in plasma reaction at room temperature and atmospheric pressure. Salicylic acid reacts with OH radical and produces 2,5-dihydroxybenzoic acid (2,5-DHBA). Then, a high performance liquid chromatography (HPLC) was carded out to detect the concentration of 2,5-DHBA. Therefore, OH radical in nonthermal plasma reaction could be calculated. In this plasma reaction, the applied voltage was controlled at 10 kV, the initial concentration of toluene was 400 mg/m3, and the gas flow rate was 300 ml/min. It was observed that when the film was placed away from the plasma area, 2,5-DHBA could not be detected by HPLC, although the sampling thne lasted for 48 h. On the other hand, when the film was placed in the plasma area and the sampling time being too long ( 4 h), the concentration of 2,5-DHBA was also below detection limit, and it could not be detected by HPLC. However, when the fihn was placed in the plasma reaction field with the sampling time being 3 h, the concentration of OH radical was calculated to be 10.54 × 1012 cm-3. In addition, concentration of OH radical was investigated under different humidity, such as 0.2%, 0.4%, 0.6%, 0.8%, and 1.0%. The results showed that the amount of OH radical stayed at order of magnitude of 1012 cm-3 and increased with the increase of humidity.