The thermal and mechanical properties for four binary blends, high-density polyethylene (HDPE)-metallocene polyethylene (MCPE), polypropylene (PP)-MCPE, poly(propylene-co-ethylene) (CoPP) -MCPE, and poly( propylene-co-ethylene-co-1-butene) (TerPP) -MCPE were investigated to compare the compatibility and molecular micromechanism of the blends. We report in this work all the blend systems that are thermodynamically immiscible but mechanically compatible which have been understood by their thermal and mechanical behaviors. A lower content of MCPE (up to 50%) in PP-MCPE, CoPP-MCPE, and TerPP-MCPE blends showed discernibly two beta transitions, whereas beta relaxation was shifted to a lower temperature with the MCPE content in the HDPE-MCPE system. These results conclude that the degree of compatibility in the HDPE-MCPE blend is the largest among the blend systems that we have studied, which also can be explained in terms of the similar chemical structure of polyolefins.