Liquid phase direct synthesis of dimethyl ether (LPDME (TM)) under various operating conditions (temperature, H-2/CO molar ratio of feed) was conducted in a mechanically agitated slurry reactor system. Each run was monitored for 60 h time on stream (TOS) in order to confirm the high activity and long-term stability of a bi-functional catalytic system (CuO-ZnO-Al2O3/H-MFI-90). Statistical experimental design was applied for determining the optimum operating conditions under which the catalytic system shows the highest performance. A significant improvement in the performance of the bi-functional catalyst was observed when the temperature and H2/CO molar ratio of feed were increased from 200 to 240 degrees C and 1 to 2, respectively at a constant pressure of 35 bar and GHSV equal to 1100 mLn/(g-cat h). CO conversion was increased from 9.1 mol% at T = 200 degrees C and H-2/CO = 1 to 79.6 mol% at T= 240 degrees C and H-2/CO = 2 and the yield and selectivity of DME also increased from 7.11 % to 47.05% and 41.57% to 59.96%, (molar basis) respectively. No significant deactivation has been observed during 60 h TOS at different operating conditions. Furthermore, from the main effect plots and response table results, it was concluded that the most effective factor on activity and stability of bi-functional catalytic system is temperature. (C) 2007 Elsevier B.V. All rights reserved.