The molecular weight distribution (MWD) of the polyolefins is an essential criterion to determine the properties of the polymer and its applications. Bimodal distribution is one of the most popular MWD in industry when melt strength besides convenient processing such as producing biaxial oriented film or pipes processing, is required. Bimodal grade is attributed to an increase in the number of entanglements and better network connectivity so that grad pretend polymer for sagging or fell down the melt strength. On the other hand during the processing of extrusion, chain scission is displayed, initially in this study polypropylene and polyethylene are used in multiple extrusion which it is causes the chain scission processes during thermo-mechanical degradation. Thus, HP525J as a bimodal polypropylene, and HyPE 35 P FA as a bimodal polyethylene grade are selected then these grades are degraded in high temperature profile and different screw speed in extruder. In the end for evaluating this phenomenon on these grades, high temperature gel permeation chromatography (HT-GPC) is employed. The MWD behavior has been measured after each turn in extruder and on the odd occasion intensity in the low molecular weight region shift to higher molecular weight region, and their MWD are compared with each other, also there are some left shifts to low molecular weight in MWD of polypropylene grade by it degradation.