Abstract: The densification mechanism of polyacrylonitrile carbon fibers during carbonization from 900 to 1 400℃ was investigated. The density, elemental composition, microstructure and weight loss of the fibers, as well as the gases released during the process were analyzed to reveal the mechanism. Results indicated that the density of the fibers was strongly dependent on the carbonization temperature and reactions involved indifferent temperature regimes. Condensation, pyrolysis and graphitization reactions were dominant at low (<1 050℃), medium (1 050-1 250℃) and high (>1 250℃) temperatures, respectively. The amount of small molecule gas released and the fiber density both increased rapidly with temperature when condensation reactions dominated. The fiber density decreased as a result of nitrogen release when pyrolysis reactions dominated above 1050℃ while the fiber density increased due to the growth and increase in order of the graphene layers during graphitization. The two fiber density maxima found with increasing carbonization temperature were attributed to the different reactions.