Water produced during methanol synthesis from a CO2-rich feed (CO2/CO/H-2) accelerated the crystallization of Cu and ZnO contained in a Cu/ZnO-based catalyst to lead to the deactivation of the catalyst. A small amount of silica added to the catalyst greatly improved the catalyst stability by suppressing the crystallization of Cu and ZnO. On the other hand, the catalysts both with and without silica were only slightly deactivated during methanol synthesis from a CO-rich feed containing a higher concentration of CO, because only a small amount of water was produced during the reaction, so no remarkable crystallization of Cu and ZnO contained in the catalyst occurred.