Abstract: 1,1-dichloro-2,2-bis(4-chorophenyl) ethylene (p,p'-DDE) is the primary metabolite of 1,1'-(2,2,2-trichloroethane-1,1-diyl) bis(4-chlorobenzene) which is a pesticide. p,p'-DDE was ubiquitous in the environment, and has significant interference effect on the male hormone receptor in the mammal. The chlorine isotopic composition (δ37Cl) of p,p'-DDE can be used to study the fate of the compound in environment. In order to obtain the optimal reaction conditions, the CuO catalysed oxidative dechlorination reaction of p,p'-DDE were conducted under vacuum, different temperatures (520-900℃) and time (0.5-1.0 h). Cl- formed in the reaction was determined by ion chromatography and the δ37Cl was determined by GasBench-IRMS. The experimental results showed there is a linear dependence between δ37Clproduct SOMC/‰ and yield of Cl- when p,p'-DDE reacted with enough CuO at 520℃ to 720℃ under vacuum condition. The linear equation was y=1.44x-4.41 with 0.96 for R2. Besides, the error value of δ37Clproduct SOMC/‰ for p,p'-DDE was lower than ±0.15‰ when the reaction conditions were 720℃ and 1h. In conclusion, a more effective and accurate preparative method was developed for the determination of δ37Clproduct SOMC/‰ of semi-volatility organo-chlorine compounds.