Effects of steam in the systems of methanol synthesis over CuZnOAl2O3 catalyst and one-step DME synthesis over CuZnOAl2O3/gamma-Al2O3-HZSM-5 were investigated, respectively. The results showed that there were two different effects in both systems: for the former (methanol synthesis), a little steam (< 10%) in the feed gas increased the catalytic activity and stability; for the latter (DME synthesis) where steam was created significantly, presence of steam in the feed gas accelerated sintering of Cu particles and deactivation velocity. The results of hydro-thermal treatment of pre-reduced CuZnOAl2O3 catalyst showed that high temperature (> 240 degrees C) and high steam content (> 10%) can greatly enhance the sintering velocity of Cu crystalline. Commercial nano-SiO2 and nano-Al2O3 with high surface area were utilized to control the steam concentration in the system of one-step DME synthesis. Owing to their hydrophilic surface, however, neither of them depressed the deactivation velocity effectively. The pre-treatment of nano-SiO2 support with alcohol can change the surface of the support from hydrophilic to hydrophobic property, which can be found to control the steam surrounding the active metallic Cu and keep the catalytic stability.