One of the main disadvantages of particle-filledcomposites based on thermoplastic matrices is a sharpdrop in the fracture strain in compositions with a fillerconcentration of about 10 vol % and higher. This isexplained by the transition from ductile to brittle frac-ture of the composite. In this paper, it is shown thatembrittlement of a particle-filled composite based on aductile polymer deforming with neck propagation canbe avoided if the following conditions are fulfilled:(i) the ultimate strength of the matrix exceeds its upperyield stress and (ii) extension of the composite is notaccompanied by the formation of diamond pores(cracks). The latter condition is satisfied when the aver-age particle size is below a certain critical value deter-mined by the fracture toughness of the polymer matrix[1].It is known that, when the filler particles are intro-duced into a thermoplastic polymer matrix, the com-posite fracture deformation decreases. If a uniformlyyielding plastic polymer is used as a matrix [2], then thefracture deformation monotonically decreases withincreasing filler content. Composites based on thematrices exhibiting neck formation upon extensionshow a substantially different behavior and, at a certaincritical concentration of the filler, they become brittle.As a result, the fracture deformation sharply decreases,by approximately two orders of magnitude [3]. How-ever, along with the materials in which the transitionfrom ductile to brittle fracture takes place, there arecomposites capable of retaining ductility in a widerange of filler concentrations [4, 5]. The aim of thisstudy was to find the conditions making it possible toavoid the embrittlement of materials based on ductilematrices deforming with the neck propagation.The experiments were performed with low-densitypolyethylene (LDPE) (15803-020 grade), medium-density polyethylene (MDPE) (F 3802 B grade), andtrans-polyisoprene (PI). As a filler, we used polydis-perse particles of vulcanized rubber based on ethylene-propylene-diene terpolymer (EPT) and isoprene rubber(IR) with an average particle size in the range from 10to 600 µm. The conditions under which the materialsbased on LDPE and MDPE matrices were prepared aredescribed elsewhere [6, 7]. Rubber particles and EPTwere blended in a press at a temperature of 100 °C. Theobtained blends were used to make 1-mm-thick platesat a temperature of 150°C and a pressure of 10 MPawith subsequent cooling under pressure. The filler con-tent was varied from 0.02 to 0.36 volume fractions(2−40 wt %).The standard dog-bone specimens for mechanicaltesting were cut from the plates and had a width of5 mm and a length of 35 mm. The tests were performedunder uniaxial tension conditions on a 203R-005 test-ing machine at an extension rate of 20 mm/min.Mechanical tests with the samples of LDPE–IR andPI–EPT composites were performed at room tempera-ture. The samples of MDPE–EPT composites weretested in a thermal chamber at a temperature of 80°C.In this case, a sample was held at the given temperaturefor 5 min prior to test.The mechanical behavior of particle-filled compos-ites was analyzed using an approach developed previ-ously [3, 8, 9], according to which three competingmechanisms of deformation in a filled composite wereconsidered: (i) neck propagation, (ii) brittle fracture,and (iii) uniform ductile yielding. Each of these mech-anisms corresponds to a certain formal parameterdepending on the filler particle concentration. The neckpropagation is characterized by the draw stress σ