High impact polystyrene (HIPS) particles with sizes well below 1 mu m have been demonstrated to interact with growing crazes giving rise to some local, non-catastrophic fibril texture damage, which increases with increasing particle size and volume fraction of the second phase. The influence of such damage on the subcritical fracture behaviour of some core-shell HIPSs, which have been well characterized from the morphological and structural points of view, is investigated. A statistical craze fibril fracture model is adopted that considers a two-parameter Weibull distribution and a Paris law for the sub-critical crack advance is derived. The meaning and dependence of the Weibull parameters on the structural and morphological characteristics of the considered materials are discussed. The proposed approach is not limited to the statistical idea and can, in principle, be used to derive the Paris behaviour in a larger class of materials.