Fourier transform infrared spectroscopy was used to characterize the structure of phenol formaldehyde (PF) resin pyrolyzed at different temperatures, and pyrolysis gas chromatography mass spectrometry was used to observe the volatiles. A pyrolysis mechanism has been consequently deduced, and several previous ambiguities have been clarified. The occurrence of carbon monoxide and carbon dioxide is attributed to the oxidization of methylene. However, not oxidization, but methylene scission is mainly responsible for the decomposition of PF resin. At the same time, methylene radicals combine with other small molecules and form some volatiles such as ethylene and methanol. With the elevation of heat treatment temperature, PF resin is progressively transformed into amorphous carbon by pyrolysis and polycyclic reactions.