The hydrogenation of CO2 to CH4 was carried out using MgO-mixed catalysts of Ru, Ni, Fe or their combinations at temperatures ranging from 80 to 150 degrees C under initial pressures of 100 Torr CO2 and 500 Torr H-2. The effect of the mechanical milling of catalysts on the reaction was investigated. It was found that (1) the yield of CH4 strongly depended on the kind of catalysts; the highest yield was achieved with Ru-MgO catalyst and the lowest yield with Ni-Fe catalyst. (2) Mechanochemically activated catalysts largely increased the production rate of CH4 and considerably decreased the activation energy of the reaction from 74 kJ/mol to 41 kJ/mol for Ru-MgO catalyst.