The fracturing of four different polyolefin materials was studied with the objective of developing a better understanding of the relationships between the morphology of the semicrystallme polymers, the morphology of their fracture surfaces and their mechanical properties. This article is focussed on the quantitative description of the fractures surfaces. The surface structure can be described in terms of self-affine fractal models, and the Hurst exponent(s) and roughness measurements can be used to describe quantitatively the fracture surface topography. Fracture surfaces generated in homopolymers can be described by a single Hurst exponent, which differs for PE and PP. For copolymers with PE and PP matrices the Hurst exponent measured on small-length scales was the same as that obtained for the matrix material, but a crossover to a second regime, with a higher Hurst exponent, was found at longer length scales. The crossover was related to the average distance between rubber particles for the PP/PE rubber phase specimen (PP-copo). The introduction of a second component seems to modify the crack propagation at long-length scales, but the propagation at shorter length scales remains unchanged. Environmental stress cracking experiments indicate that each regime can be related to brittle or ductile fracturing processes.