Cu/g-Al2O3 catalysts for methanol synthesis from CO2 are prepared and modified by metal oxides via impregnation in the present work. Results indicate that promoters modification leads to the formation of small Cu0 particles with a high dispersion, improves catalytic performance of Cu-based catalysts in methanol synthesis. Moreover, the activation temperature is another important factor on affecting the Cu0 dispersion and particle size. The investigation on CO2 hydrogenation shows that methanol is mainly from hydrogenation of activated CO2 with active hydrogen on Cu0 particles, which is inhibited by high reaction temperature due to improvement on reverse water-gas shift reaction and methanation.