Blends of polypropylene (PP) with a copolymer of ethylene and octene (EOC) were studied. The effect of viscosity ratio of elastomer domains to the matrix PP eta(d)/eta(m) (where subscript d and m designate dispersed phase and matrix, respectively) on fibril formation was investigated. Fibril morphology was observed for both eta(d)/eta(m) approximate to 1 and eta(d)/eta(m) > 1 (eta(d)/eta(m) = 1.70 or 2.40). As the viscosity ratio increased further, the spherical, elliptical, and rodlike morphology appeared. Fibril formation at > 1 is possibly attributed to the effect of the interface interaction between the two phases. With soft fibril formation of EOC in PP, mechanical properties of the blends such as tensile and flexural properties were not improved and the impact strength was low compared to those of blends with spherical, elliptical, and rodlike dispersed morphology.