Polyethylene has been widely applied in our society and is known to burn well. Many researchers have been studying to control the flammability of polyethylene when adding several catalysts. However, this study has confirmed the effect of the molecular weight on the flammability of polyethylene. This article displays combustion states of two different molecular weights of polyethylene. The low molecular weight of polyethylene did not ignite and the high molecular weight of polyethylene ignited and burned well. From this result, it was difficult for polyethylene to continue combustion under limited conditions, such as the vertical configuration of the materials, air circumstance, and low molecular weight. To analyze such behaviors, the relationship between combustion state and degradation behavior of polyethylene was compared to that of polypropylene (PP) and polystyrene. PP and polystyrene decomposed into small molecules directly through chain-end scission and that low-molecular weight scission products went to gas phase and reacted with oxygen and consequently fire around specimens during combustion. Polyethylene decomposed into relatively higher molecular weight scission products through random scission at primary degradation. The scission products further decomposed into lower molecular weight products with drip, which caused the combustion under specimens. There were no big differences in a weight loss temperature and scission products at high temperature between a low molecular weight of polyethylene and a high molecular weight of polyethylene. Therefore, the degradation and combustion behavior of scission products of polyethylene in primary degradation warrant further research on flame retardant of polyethylene. (c) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 436-443, 2011