Many of Plastic additives compound can be used for multiple functions, and many of them interact synergistically to protect against heat, light, and oxidation. Antioxidants are used in a variety of resins to prevent oxidative degradation. Degradation is initiated by the action of highly reactive free radicals caused by heat, radiation, mechanical shear, or metallic impurities. Stabilization of polyolefins is necessary to protect the polymer from degradation during melt processing and conversion. For most applications, a combination of a phenolic antioxidant and a phosphite melt processing stabilizer can provide the primary stabilization necessary to provide the retention of physical properties, good processability, and long-term thermal stability without compromising the overall aesthetic appearance of the products. Tris-nonylphenylphosphite (TNPP) is a successful phosphite stabilizer for PE grades. In this research, the effects of Irgafos 168, as a phosphite secondary antioxidant, and TNPP, as currently used phosphite antioxidant, on the degradation behavior of High (HDPE), Medium (MDPE) and Linear Low Density Polyethylene (LLDPE) grades were compared and investigated. Thermal stability of samples obtained by study of changes in Yellow Index (YI) and Melt Flow Index (MFI) for all samples, determined after four times extrusion. Rheological behavior of prepared samples were also studied. As showed the YI results, most of samples preserved colors after four times extrusion and according to MFI results, changes in MFI results after four times extrusion were limited, so that indicates the performance of antioxidant combination for these samples are acceptable. The results shows that applying a particular and appropriate combination of phenolic/Irgafos 168 antioxidant package for each of PE grades was effective enough to take TNPP place in our produced samples. Rheological behavior of produced samples also showed the same results.