Pyrolysis kinetics of oil shale mixed with high-density polyethylene (HDPE) was investigated using a thermogravinietric analysis (TGA) system at various heating rates of 2, 10, 20 and 50 K/min in the temperature range of 300-1273 K in the nitrogen atmosphere. Kissinger-Akahira-Sunose, Friedman, Flynn-Wall-Ozawa and Coats-Redfern methods have been used to determine activation energies of materials degradation. The analysis of the process mechanism by Criado and Coats-Redfern methods showed the following: the mechanism of thermal degradation process of HDPE is describable by the "Contracting cylinder" model (R2 mechanism); and the most probable model for the pyrolysis process of oil shale kerogen is the diffusion model (D4 mechanism), while the mixture degrades following the kinetic model of D4. It has been found that during thermal decomposition of oil shale/HDPE mixture no significant interaction of solid-phase components had taken place under the experimental conditions investigated.