: In this study, the effect of quercetin, gallic acid and hesperidine as a natural stabilizer on the melt stability of high density polyethylene and linear low density polyethylene was studied in presence and absence of secondary stabilizer (Irgaphos 168). This study was conducted with polyethylene of typeZigglernata. Heavy polyethylene samples containing different amounts (1000, 500, 300 ppm) of natural stabilizers in presence and absence of secondary stabilizer was prepared by extrusion process using a double-screw extruder. Linear Low Density Polyethylene samples containing 300 ppm of natural stabilizer with and without secondary stabilizer was prepared using single-screw extruder. All samples are six times sequentially extruded. Irganox 1010 was used as a synthetic phenolic stabilizer. Measuring of properties such as rheological, thermal stability and yellow index of all specimens were performed using standard methods. Measuring results of Melt Flow Index (MFI) showed that the presence of natural stabilizers in heavy polyethylene prevents crosslinked reactions and causes lower decrease in melt flow rates at the first phase of extrusion. Also the presence of these stabilizers in Linear Low Density Polyethylene of chain scission reactions have a better effect than synthetic stabilizers and increasing in the Melt Flow Index is less than that of Irganox 1010. Examination of capillary tube rheometer results and the RMS rheometer results showed that the increase in viscosity and molecular weight due to crosslink mechanism and chain branching during the molten process of heavy polyethylene containing natural stabilizers are less visible. The results of thermal stability properties showed that the melt stability of keotesetin and gallic acid compared to the synthesized stabilizer studied in heavy polyethylene and linear low density polyethylene is better and even its stability is enhanced by adding a second stabilizer. The difference in performance of natural and synthetic stabilizers can be due to the differences in their OH radical groups. On the other hand, examination of the yellow index showed that using of natural stabilizers can change the color of the polyethylene. However, despite this weakness of the natural stabilizers, it can be used to prevent thermal degradation in cases where the color of the product falls in second order. Examinations of rheological properties and thermal stability of galic acid and kerostin stabilizer at different concentrations showed that using of 300 ppm of these stabilizers provides acceptable melt performance. Keywords: Natural Stabilizer, Viscosity, Polyethylene, Melt processing, Thermal degradation