The reaction involving radical species is a key factor during the pyrolysis process used to form hydrocarbons. In this study, electron paramagnetic resonance (EPR) spectroscopy was used to characterize the properties of free radicals in the pyrolysates of oil shale. Effects of temperature on the yields and the EPR properties of thermal bitumen, shale oil, and semicoke were investigated, with the aim to understand the behavior of free radicals during the oil shale pyrolysis process. This study shows that the free radical concentrations (Ng) of shale oil and semicoke become higher with increasing temperature. The yield and Ng of thermal bitumen as an intermediate product follow the similar trends in the whole temperature range, first increasing and then decreasing. This is attributed to the competing mechnism of thermal bitumen generation and decomposition. The Ng of shale oil is lower than those of semicoke and thermal bitumen due to the coupling reaction of free radicals before the volatiles being condensed. The g-values and linewidths of thermal bitumen, shale oil, and semicoke are also affected by temperature, revealing the changing chemical structure and the surrounding environment of free radicals during the pyrolysis process.