This paper presents the decomposition by-products of trifluro-iodo-methane and their relative proportions in the gas phase under the occurrence of partial discharge. The experiment was performed in the presence of water vapor from 250 to 400 ppm under a non-uniform electric field configuration. The experimental results reveal that the by-products of C 2 F 6 , C 2 F 4 , C 2 F 5 I with the amount of 1300, 200, and 55 (CH 3 I) ppm, respectively, were produced for a cumulative charge of 161 mC. Other by-products, such as C 3 F 8 , CHF 3 , C 3 F 6 CH 3 I were obtained at less than 30 ppm C 2 F 6 was the dominant gas by-product of trifluro-iodo-methane suffering partial discharge.