The waste plastics can be converted into valuable fuels by the process of pyrolysis. To optimize the process of pyrolysis of polymers, the knowledge of thermal degradation kinetics is needed and it is usually studied by thermogravimetry. Thermogravimetric analysis is an excellent tool for studying the kinetics of thermal degradation, since it enables determination of the basic kinetic parameters such as activation energy, reaction order and pre-exponential factor. In this work, kinetic analysis of thermal and catalytic degradation of polyolefin mixture of polypropylene and high-density polyethylene was investigated under non-isothermal conditions at different heating rates: 3–20 K/min. The activation energy was determined applying the model-free methods, proposed by Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose. Results have shown that the average activation energy values determined by both methods are similar. The activation energy for degradation of the polyolefin mixture decreases considerably by adding the catalyst.