In the recent years, blending of different polymers is receiving increasing attention from researchers for various reasons including the possibility of creating a material or product for new and more industrial applications to meet specific processing and performance requirements that cannot be satisfied by a single component. Polyethylene (PE) and polypropylene (PP) and their blends have attracted a lot of attention due to their potential industrial applications such as piping systems in pressure vessels and pipelines. The main objective of this work is to study the effect of the thermal treatment/aging and PE/PP blending ratio (composition range) on the mechanical behaviour (tensile and hardness) of PE, PP and PE/PP blends. Samples of PE/PP blends containing 100/00, 75/25, 50/50, 25/75 and 0/100 weight percentage were prepared via injection molding technique and thermally treated/aged at 100 °C for 0, 2, 4, 7, 14 days. The tensile measurements indicated that the yield strength and the modulus decrease with increasing PE content. It was also observed that PE, PP and their blends deform in ductile modes. They undergo a uniform yielding over a wide range of deformation, which is followed by strain hardening and then failure. The strain to break for pure PE is found to be much higher than that for pure PP and for their blends, intermediate values have been observed. The hardness measurements have also revealed that increasing PE content in PE/PP blends reduced the hardness value of PP, however thermal aging hasn't affected the hardness showing a good correlation with the tensile properties.