The effect of time-temperature treatment on morphology of polyethylene-polypropylene (PE-PP) blends wasstudied to establish a relationship between thermal history, morphology and mechanical properties. Polypropylene (PP)homopolymers were used to blend with various polyethylenes (PE), including high density polyethylene (HDPE), lowdensity polyethylene (LDPE), linear low density polyethylene (LLDPE), and very and ultra low density polyethylene(VLDPE and ULDPE). The majority of the blends were prepared at a ratio of PE:PP = 80:20, while blends of PP and LLDPEwere prepared at various compositions. Thermal treatment was carried out at temperatures between the crystallizationtemperatures of PP and PEs to allow PP to crystallize first from the blends. On cooling further, PE crystallized too. A verydiffuse PP spherulite morphology in the PE matrix was formed in some partially miscible blends when PP was less than 20%by mass. Droplet-matrix structures were developed in other blends with either PP or PE as dispersed domains in a continuousmatrix, depending on the composition ratio. The scanning electron microscopy (SEM) images displayed a fibrillar structureof PP spherulite in the LLDPE-PP (80:20) and large droplets of PP in the HDPE-PP (80:20) blend, providing larger surfacearea and better bonding in the LLDPE-PP (80:20) blends. This explains why the blends with diffuse spherulite morphologyshowed greater improvement in tensile properties than droplet-matrix morphology blends after time-temperature treatment. The effect of time-temperature treatment on morphology of polyethylene-polypropylene (PE-PP) blends was constructed to establish a relationship between thermal history, morphology and mechanical properties. Polypropylene (PP) homopolymers were used to blend with various polyethylenes (PE) High density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and very and ultra low density polyethylene (VLDPE and ULDPE). The majority of the blends were prepared at a ratio of PE: PP = 80 : 20, while blends of PP and LLDPEwere prepared at various compositions. Thermal treatment was carried out at temperatures between the crystallizationtemperatures of PP and PEs to allow PP to crystallize first from the blends. On cooling further, PE crystallized too. A verydiffuse PP spherulite morphology in the PE matrix was formed in some partially misble blends when PP was less than 20% by mass. Droplet-matrix structures were developed in other blends with either PP or PE as dispersed domains in a continuous matrix, according to the composition ratio. The scanning electron microscopy (SEM) images were a fibrillar structure of PP spherulite in the LLDPE-PP (80:20) and large droplets of PP in the HDPE-PP (80:20) blend, providing greater surfacearea and better bonding in the LLDPE-PP (80:20) blends. This explains why the blends with diffuse spherulite morphologyshowed greater improvement in tensile properties than droplet-matrix morphology blends after time-temperature treatment .