A method of preparing graphite fluoride such as (CF).sub.n or (C.sub.2 F).sub.n by heterogeneous contact reaction between a carbon material such as graphite or petroleum coke and fluorine gas at about 200.degree.-550.degree. C. In the gas phase of the reaction system, the total concentration of higher fluorocarbons having more than four carbon atoms formed by side reactions is controlled so as not to become above 3% by volume by, for example, condensation or catalytic decomposition of at least a portion of the higher fluorocarbons in the gas flowed out of the reaction chamber for recirculation. Such control is highly effective for prevention of rapid and violent decomposition of the graphite fluoride existing in the solid phase of the reaction system induced by sudden decomposition of the higher fluorocarbons in the reaction system to lower fluorocarbons.