Natural and modified clays based on montmorillonite structure are largely used to preparation of polymer nanocomposites. However, these clays can also cause changes of properties of the polymer matrix after thermal treatment. The aim of the present study was to evaluate the changes of properties of the linear low-density polyethylene (LLDPE), polypropylene (PP) and LLDPE/PP blend after thermomechanical processing on high shear rates. Thus, a natural bentonite clay in powder form was previously incorporated at 5 wt% to the LLPDE, PP and LLPDE/PP blend in the proportion 70/30 wt%, respectively. Afterward, the materials were processed in an internal mixer MH-600 at 4500 rpm under nitrogen flow, maintaining the shearing by 90 s after the polymer melting. Figure 1 presents the results of melt flow rate (MFR) of the materials and references (polymers without clay and non-processed) performed at 190 ºC and 2.16 Kg of load (LLDPE and blends) and 230 ºC/2.16 Kg for PP, according to standard practice ASTM D 1238. Non-processed LLDPE presents MFR of 0.94±0.01 g/10min and after thermomechanical processing without clay the MFR it decreases to 0.50±0.03 g/10min. On the other hand, the MFR of the LLDPE containing natural clay was 3.6±0.6 g/10min. The changes in MFR should be caused by structural modifications, which are enhanced by clay action. In LLDPE/PP blend the MFR is comparatively higher than the LLDPE with and without clay, respectively. This is due to the presence of tertiary carbon in PP, causing higher sensibility to the degradation. The sensibility to the degradation of the PP can also be verified by the high MFR (above 14 g/10 min) for PP processed with and without clay. The natural clay significantly changes the MFR of the polyolefins LLDPE, PP and LLDPE/PP blends when processed under high shear rates due to structural changes probably caused on the polymer. The use and control of these changes can be useful for polymer processing on conditions of difficult separation of the polymer mixture, such as mechanical recycling operations.