Several blends, covering the entire range of compositions, of a metallocenic ethylene-1-octene copolymer (CEO) with a multiphasic block copolymer, propylene-b-(ethylene-co-propylene) (CPE) [composed of semicrystalline isotactic polypropylene (iPP) and amorphous ethylene-co-propylene segments], have been prepared and analyzed by differential scanning calorimetry, X-ray diffraction, optical microscopy, stress-strain and microhardness measurements, and dynamic mechanical thermal analysis. The results show that for high CEO contents, the crystallization of the iPP component is inhibited and slowed down in such a way that it crystallizes at much lower temperatures, simultaneously with the crystallization of the CEO crystals. The mechanical results suggest very clearly the toughening effect of CEO as its content increases in the blends, although it is accompanied by a decrease in stiffness. The analysis of the viscoelastic relaxations displays, first, the glass transition of the amorphous blocks of CPE appearing at around 223 K, which is responsible for the initial toughening of the plain CPE copolymer in relation to iPP homopolymer. Moreover, the additional toughening due to the addition of CEO in the blends is explained by the presence of the beta relaxation of CEO that appears at about 223 K.