Introduction of a small fraction of large particles often embrittles a polymer, and the rupture of the polymer occurs at small relative elongation. The sharp loss of the deformability of the composite is caused by the appearance of so-called diamond-shaped pores [1] observed previously in [2, 3]. It was shown that the size of particles responsible for the appearance of diamondshaped pores is determined by the critical crack opening and, therefore, by the breakdown viscosity of the matrix polymer. Rupture of particles or their separation from the matrix under tension gives rise to the formation of pores whose shape is determined by the size of particles [1]. Small and large particles form oval pores and diamondshaped pores, respectively. With further tension, the two types of pores that appear behave differently. An oval pore develops only along the material-elongation direction. A diamond-shaped pore grows in three directions, parallel and perpendicular to the sample tension axis, in particular, along the sample thickness, which leads to early failure. In polymers deformed by the propagation of a neck, the problem is compounded, because the growth of pores is often localized in the narrow formed neck. As a result, the material breaks down at small macroscopic strain. Although the fracture process (growth of diamond-shaped pores) is typically plastic at the mesoscale, the material behaves as a brittle material at the macroscale. This work aims to determine the dependence of the critical size of particles at which diamond-shaped cracks appear on the properties of the polymer matrix. Lukoten F 3802 medium-density polyethylene, Lipol A4-70 polypropylene, and 168030-070 low-density polyethylene are used for composites. Polymers were filled with powdered-rubber particles with sizes from 50 to 600 μm. A monodisperse filler was obtained by grading the polymers into grain sizes with a standard set of sieves. Each polymer was mixed with rubber particles in a single-screw laboratory extruder. The filler