This paper demonstrates how to derive Arrhenius parameters as well as reaction model of pyrolysis reaction of high density polyethylene (HDPE) from isothermal kinetic data. We introduced a state-of-the-art thermobalance that enables to record a weight decrease with time under pure static condition. As a result of comparing experimental reduced time plots (RTPs) with theoretical ones, the pyrolysis reaction of HDPE accommodates "contracting cylinder" model, allowing its model to be 2(l -alpha)(1/2). The reaction model derive here was used to evaluate each Arrhenius parameters from isothermal and non-isothermal data. A couple of evidences strongly support the properness of the reaction model derived from RTP and the corresponding Arrhenius parameters.