Plasma application for environment improvement is desirable. In order to improve the efficiency of toxic gas decomposition by plasma, it is worthwhile to clarify the behavior of radicals in non-thermal plasma. Although previous studies have revealed the behavior of radical densities and temperatures in atmospheric discharge, they are not yet connected to apprehension or simulation of practical toxic gas decomposition by atmospheric plasma. In this study, the coaxial dielectric barrier discharge (DBD) cylinder reactor is prepared, where the radical behavior and toxic gas decomposition efficiency can be measured all together. The density and temperature of OH radical are directly measured by laser-induced fluorescence (LIF), and trichloroethylene (TCE) decomposition efficiency is also investigated. OH density measurement indicated that OH density is about 20 ppm at 3 μs after discharge when 28 kV is applied. Besides, the acceleration of OH decay with increasing TCE is observed, which indicates that OH reacts with TCE. On another front, TCE decomposition efficiency by AC DBD is also investigated in the same reactor. With the characteristics and humidity dependence of OH behavior measured above, we can simulate TCE decomposition efficiency dependence on humidity.