Abstract <p>The article presents the results obtained in the study of the influence of UHMWPE fibers activation by nonequilibrium low-temperature plasma of a radio-frequency capacitive discharge at reduced pressure and direct fluorination on the properties and failure mode of the fibers and composite materials based on them. The application of activated UHMWPE fiber for reinforcing epoxy and epoxy-urethane matrices makes it possible to produce light-weight composite materials (CM) with high physicomechanical indices. The interaction between the fiber and the matrix during the production of CM can be increased both by activating the fiber and by choosing the composition of the matrix. The plasma treatment and direct fluorination appeared to be particularly effective for study of the effect of fiber activation on the bending and shear properties of the composite, thus demonstrating a high increase in the interaction at the fiber/matrix interface. After the plasma treatment of SK-75 fibers, the composites based on Epicot-828 and EPUR matrices had increased bending strength σ_b by a factors of 3 from 155 to 470 MPa and 2.96 (about 3) from 145 to 430 MPa, respectively. The shear strength τ_sh increased by the factors of 1.66 from 12 to 20 MPa and 1.57 from 18 to 30 MPa for the composites reinforced with plasma-activated SK-75 fibers based on Epicot-828 and on EPUR matrices respectively. The composite shear τ_sh and bending strength σ_b increased by the factors of 2.2 from 10 to 22 MPa and 2 from 160 to 320 MPa for CM based on Epicot-828 reinforced with activated SK-75 fibers by direct fluorination. Activation of the fiber with the help of plasma and fluorination allows оne to make polyethylene plastic a monolithic composite. The failure mode of such composites points to the high strength of the bond between the fiber and the matrix.</p>