Two sets of thermal reaction experiment for chlorinated hydrocarbons were performed using an isothermal tubular-flow reactor in order to investigate thermal decomposition, including product distribution of chlorinated hydrocarbons. The effects of H2 or Ar as the reaction atmosphere on the thermal decomposition and product distribution for dichloromethane(CH2Cl2) was examined. The experimental results showed that higher conversion of CH2Cl2 was obtained under H2 atmosphere than under Ar atmosphere. This phenomenon indicates that reactive-gas H2 reaction atmosphere was found to accelerate CH2Cl2 decomposition. The H2 plays a key role in acceleration of CH2Cl2 decomposition and formation of dechlorinated light hydrocarbons, while reducing PAH and soot formation through hydrodechlorination process. It was also observed that CH3Cl, CH4, C2H6, C2H4 and HCl in CH2Cl2/H2 reaction system were the major products with some minor products including chloroethylenes. The CH2Cl2/Ar reaction system gives poor carbon material balance above reaction temperature of 750℃. Chloroethylenes and soot were found to be the major products and small amounts of CH3Cl and C2H2 were formed above 750℃ in CH2Cl2/Ar. The thermal decomposition reactions of chloroform(CHCl3) with argon reaction atmosphere in the absence or the presence of CH4 were carried out using the same tubular flow reactor. The slower CHCl3 decay occurred when CH4 was added to CHCl3/Ar reaction system. This is because :CCl2 diradicals that had been produced from CHCl3 unimolecular dissociation reacted with CH4. It appears that the added CH4 worked as the :CCl2 scavenger in the CHCl3 decomposition process. The product distributions for CHCl3 pyrolysis under argon bath gas were distinctly different for the two cases: one with CH4 and the other without CH4. The important pyrolytic reaction pathways to describe the important features of reagent decay and intermediate product distributions, based upon thermochemistry and kinetic principles, were proposed in this study.