Abstract: We have studied a series of CoMo hydrodesulfurization(HDS) catalysts supported on TiO2-Al2O3 composite oxide and Al2O3. The difference of these catalysts was investigated by TPR(Temperature Programmed Reduction), FT-Raman, and HRTEM (High Resolution Transmission Electron Microscopy) techniques. The HDS tests were performed in a high pressure fixed bed micro-reactor with 4,6-DMDBT(4,6-Dimethyldibenzothiophene) as a probe molecule. TPR spectra results indicated that the interaction between MoO3 and TiO2-Al2O3 composite support was weaker than MoO3 and Al2O3 support, which resulted in more octahedral Mo species (MoⅥ) forming on the surface of support. Raman spectra showed that the existence of TiO2 in TiO2-Al2O3 composite support made the highest band of MoOx species shift from 958 cm-1 in MoO3/ Al2O3 to 965 cm-1 in MoO3/ TiO2-Al2O3. The blue-shift of this band which was attributed to terminal Mo=O of Mo7O6－24 implicated that the degree of aggregation of polymeric molybdenum-oxide units had increased. HRTEM graphs showed there were more multilayer MoS2 clusters in catalyst supported on TiO2-Al2O3 composite support. And multilayer MoS2 clusters were considered to be more active in HDS. The HDS tests of 4,6-DMDBT showed that the catalyst supported on TiO2-Al2O3 had higher activity than on Al2O3.