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 for polyethylene grades. Tris-nonylphenylphosphite (TNPP) is a successful phosphite stabilizer for polyethylene (PE) grades due to its excellent price/performance ratio and its good compatibility in PE. As a response to ecological concerns surrounding some chemistries with similarities to nonylphenol, some organizations have taken cautious step of removing these stabilizers. This has created a need for resin producers to devise new environmentally friendly formulations which are able to meet the requirements of these products without containing TNPP. In this research, the effects of Irgafos 168, as a phosphite secondary antioxidant, and TNPP on the degradation behavior of High (HDPE), Medium (MDPE) and Linear Low Density Polyethylene (LLDPE) grades were compared and investigated. For each grade, five samples with different amounts of antioxidant additives combination were prepared and characterized. Thermal stability of samples obtained by a capillary Rheometer and study of changes in Oxygen Induction Time (OIT) and Melt Flow Index (MFI). As showed OIT results, for HDPE and MDPE samples, utilize the replacement additive, improved thermal degradation behaviour in most of samples, but for LLDPE samples, improvement was achieved by used Irgafos 168 up to a specified amount 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. Rheological data confirmed the same results. The results shows that applying a particular and appropriate combination of phenolic/Irgafos 168 antioxidant package for each of polyethyle grades was effective enough to take TNPP place in our preduced samples.