In this study, to solve the problems of previous techniques, a new thermal degradation method was used for producing polyetylene wax having a low polydispersity index. The thermal degradation method was characterized by putting the preheating stage before the reaction stage. At the preheating stage, to increase the thermal conductivity of raw material, the raw material was heated up to a certain temperature(250-370℃), which was lower than the reaction temperature(380-400℃), and was remained at that temperature for a certain time. Polyethylene wax with a low polydispersity index(less than 3.90) could be produced within 5.40 hr using both the thermal degradation method and an external-loop reactor in which specially designed impellers were installed for more effective heat transfer. The effect of the temperature of preheating stage on the polydispersity of product, the viscosity of medium and the manufacturing time was negligible. An increase in the number of preheating stage only marginally affected the polydispersity index of product. When the reaction temperature increased, the viscosity of medium was more rapidly decreased with time and, hence, the reaction time was decreased. The viscosity of medium was decreased with increasing the reaction time.