1. Fracture of silumins in static loading starts with the formation of voids around the silicon particles. The size of the voids is comparable with the size of of the particles, which is responsible for the close relationship of the fracture relief to the degree of dispersion of the structure. 2. A fatigue crack, originating in the wide slip bands, propagates preferentially in the solid solution and therefore the fatigue zones of fractures of samples of alloys with different degrees of dispersion of the structure differ little in roughness. 3. In the modified and unmodified alloys in fatigue crack development, striations are formed. The distance between them indicates that the rate of fatigue crack propagation in the modified silumin is greater than in the unmodified.