The focus of this study is to investigate the sintering mechanism of polyethylene. Experiments were conducted to observe the high temperature sintering behaviors of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE). It was found that the sintering of polyethylene follows Frenkel＇s model and is basically a Newtonian viscous flow. The sintering rate decreases as the viscosity of the resin increases. By increasing the temperature, one can shorten the time needed to complete the coalescence process. Experimental investigation will provide an improved understanding of the sintering of semi-crystalline polymers.