Abstract The mechanism of thermal decomposition of graphite fluoride, (CF)n in a vacuum was studied by means of chemical and gas analyses, and the measurement of crystallinity. The crystallinity of (CF)n was slightly improved in the step of initial decomposition. The fluorine/carbon atomic ratio of initial gaseous product evolved by decomposition was about 2. The product is CF2 or C2F4, and then it gives several compounds by secondary reaction. Every second layers of (CF)n are decomposed, and the evolved molecules or radicals abstract fluorine atoms which are bonded to the neighboring two layers.