Polypropylene (PP) is an affordable plastic commodity but lacks massive use in engineering applications given its limited mechanical properties including significantly low impact strength. In this work, three polyolefin-based copolymers including ethylene-octene random block (Engage, E), ethylene-octene multi-block (Infuse, I), and ethylene-propylene copolymer (Versify, V) were blended with PP. It was found that the V copolymer was miscible while the rests were immiscible in the PP matrix. Mechanical testings indicated that the addition of the 20% E and 20% I copolymers significantly enhanced the impact toughness of PP up to 83% and 108% at −20°C, respectively. At a higher temperature such as 25°C, the impact toughness of the PP/E20 and PP/I20 were 400 and 571%, respectively. The enhancement of the PP impact toughness was governed by the particle size of the added immiscible copolymers. Experimental results also revealed that the good compatibility between PP and copolymers has an insignificant influence on the mechanical properties of PP while the essence of added copolymer plays a key role instead. The findings from this work provide significant insights into the design of PP-based products with desirable properties to satisfy requirements demanded by engineering applications.