The real-time gas mass filter system consisting of a mass filter and a capillary sampling tube was developed for the analysis of products during electron beam (EB) irradiation of trichloroethylene (TCE)air mixture. Interesting trace substances in gases are analyzed by this gas mass filter system in real time. The gases at atmospheric pressure are introduced to the mass filter under vacuum through a capillary tube without packing. The system was calibrated with three different standard mono-gases which contain known concentrations of sulfur dioxide, benzene and chlorobenzene for each. And its detectable limits for each gas were in the range of 0.7-1 ppmv. Products of irradiated TCE-air mixture were analyzed with the system in real time. The concentrations of dichloroacetyl chloride and carbonyl chloride (COCl2) increased by low dose irradiation when TCE was decomposed. These products decreased by higher dose irradiation and were identified as primary products. Trichloroethylene and these primary products were oxidized into CO2, Cl-2, and HCl at 15 kGy. Carbonyl chloride was dissolved in NaOH aq for natural-oxidation into CO32- and Cl-. The doses for the complete oxidation of TCE and the products were decreased from 15 to 7 kGy by the combination of the irradiation and the dissolution of the irradiated gas. The decomposition mechanism of TCE, especially formation of COCl2, was clarified from the change of the products as a function of dose.