The termal decomposition of low density polyethylene(LDPE) and 25% ~ 48% chlorinated(CPE)were studied using a dynamic thermogravimetry in the stream of nitrogen gas with 20ml/min. The mathmatic method, differential and Integral method were used to obtain value of activation by the above methods agree with each other very well. The maximum average activation energy calculated was 78.1kcal/mol. The thermal decomposition of LDPE and CPE were considered to bl carried out by main chain scission and the thermogravimetric trace curve agree with the theoretical equation.