The pyrolytic reaction of 1,1-dichloroethylene(CH2CCl2) has been conducted to investigate thermal decomposition of chlorocarbon and product formation pathways under hydrogen reaction environment. The reactions were studied in a isothermal tubular flow reactor at 1 atm total pressure in the temperature range 650~900 ℃ with reaction times of 0.3~2.0 sec. A constant feed molar ratio CH2CCl2:H2 of 4:96 was maintained through the whole experiments. Complete decay(99%) of the parent reagent, CH2CCl2 was observed at temperature near 825 ℃ with 1 sec. reaction time. The important decay of CH2CCl2 under hydrogen reaction environment resulted from H atom cyclic chain reaction by abstraction and addition displacement. The highest concentration (28%) of CH2HCl as the primary product was observed at temperature 700 ℃, where up to 46% decay of CH2CCl2 was occurred. The secondary product, C2H4 as main product was detected at temperature above 775 ℃. The one less chlorinated ethylene than parent increase with temperature rise subsequently. The HCl and dechlorinated hydrocarbons such as C2H4, C2H6, CH4 and C2H2 were the main products observed at above 825 ℃. The important decay of CH2CCl2 resulted from H atom cyclic chain reaction by abstraction and addition displacement. The important pyrolytic reaction pathways to describe the features of reagent decay and intermediate product distributions, based upon thermochemical and kinetic principles, were suggested.