The CO2 decomposition utilizing mechanically milled wustite powders was qualitatively and quantitatively examined and its mechanism was investigated. The wustite phase is stable at least up to 6 h of milling, and the lattice parameter, the crystallite size and the average particle diameter are monotonously decreased with milling time, while the BET specific surface area is correspondingly increased. The mechanically milled FeO powder decomposes CO2 into graphite and amorphous carbon at 773 K, where the decomposition intensity increases with milling time, while unmilled FeO decomposes CO2 into CO with the same annealing condition. It is found that the FeO powder thermally decomposes into Fe and Fe3O4 prior to the reaction with CO2, followed by the precipitated Fe reacting with CO2, and also that the thermal decomposition is promoted by the milling process.