The mechanical properties of isotactic polypropylene (iPP) and propylene-ethylene block copolymer (Co-PP) with carbon black (CB) were added as a filler. By mixing appropriate amounts of the two components through melt-blending in a twin-screw extruder, the blended pellets were prepared to a series of test specimens by injection molding. A scanning electron microscopic study was performed of the morphologies of the impact fractured surfaces. The blending of CB in Co-PP not only improves the impact strength, but also improves the flexural modulus and tensile strength; however, the heat distortion temperature (HDT) of the Co-PP/CB blends decreased with greater filler content. Furthermore, the filler of CB improves the tensile yield strength only at low filler content in iPP/CB blends, and the heat distortion temperature (HDT) and flexural modulus of the iPP/CB blends increased with greater filler content. The impact behavior is not good for the iPP/CB blends. Overall, Co-PP/CB has better interaction of molecules than iPP/CB.