CaCO3-polyethylene (PE) compositions, containing an ultrahigh molecular polyethylene (UHMPE) interlayer between the filler surface and the PE matrix, were synthesized by two-step polymerization of ethylene on a filler surface activated with a suitable catalyst. The properties of the compositions were studied depending on the molecular weight of the PE matrix and the thickness of the UHMPE intermediate layer at the filler particles. It was shown that the presence of UHMPE as an interlayer in chalk-UHMPE-PE compositions leads to an increase of plastic deformation of the materials as long as the M. value of the PE matrix is higher than is the brittleness threshold for PE. Chalk-UHMPE-PE compositions exhibit a higher ability for plastic deformation compared to chalk-PE compositions based on a PE matrix of a molecular weight equal to the molecular weight of the total polymer phase (UHMPE-PE) in the first case. There is no improvment of the mechanical properties when the UHMPE is dispersed in the compositions and not as an interlayer between a filler and a matrix. This means that the method of polymerization filling allows one to incorporate the polymer interlayer with a desired nature and properties between a filler surface and polymer matrix in filled polyolefin compositions.