Abstract: Dimethyl ether (DME) is one of the best indirect liquefied products of coal. It can be used not only as the automobile fuel substitute for diesel engine, but also as the household fuel substitute for LPG. Because the oil resources in China can not meet the needs of the fast economic growth, DME is more likely to become a new generation of clean fuel. For DME synthesis in slurry phase, the stability of the catalyst is a key problem. For better understanding the deactivation mechanism of the catalyst, the experiments were designed as several individual reactions involved in the one step synthesis process, such as methanol synthesis, methanol dehydration and water gas shift reaction. Each reaction was carried out at the simulated reaction conditions like DME synthesis. In this paper, γ-alumina modified by V2O5 or Sm2O3 as the dehydration component of the composite catalyst for dimethyl ether synthesis was prepared by incipient wetness impregnation method. The DME synthesis catalysts were obtained by physically mixing the methanol catalyst and the equal granular size modified γ-Al2O3 at a weight ratio of 2∶1. The DME synthesis performances of the composite catalysts were evaluated in an agitated autoclave slurry reactor. Effluent gases were analyzed by on-line gas chromatographs using carbon sieve column and a thermal conductivity detector (TCD) for hydrogen, carbon monoxide and methane, GDX-401 column and a flame ionization detector (FID) for hydrocarbons, methanol and DME. The catalysts were characterized by H2-TPR, Pyridine-IR, reductive NH3-TPD and XRD. The H2-TPR results showed that CuO in the composite catalysts can be easily reduced with addition of V2O5 or Sm2O3. PyridineIR and reductive NH3-TPD indicated that the amounts of Brnsted acidic sites and Lewis acidic sites change much less when the γ-Al2O3 is modified. The ratio of the weak acidic sites to the strong acidic sites increases with the introduction of V2O5 and Sm2O3. XRD patterns revealed that either V2O5 or Sm2O3 disperses well on γ-Al2O3 and there is no new compound formed. In DME synthesis reaction, it was found that the γ-Al2O3 modified by V2O5 or Sm2O3 shows higher activity and DME selectivity compared to the unmodified one.