Abstract: Multi-walled carbon nanotubes (MWCNT) were modified by concentrated nitric acid and the effect on surface acidity, dispersion of Co-Mo catalysts on them, and hydrodesulfurization (HDS) activity were respectively characterized by temperature-programmed desorption (TPD) and Boehm and mass titration, XRD and a flow micro-high pressure reactor using dibenzothiophene (DBT) as the sulfur-containing model compound. It is found that MWCNT is slightly basic and that modified MWCNT shows strong acidity with total amount of acid for 6 and 24h modified samples, M-MWCNT-6 and M-MWCNT-24 being 1.29×10-3 and 2.46×10-3 mol/g respectively. The dispersion of Co-Mo catalyst on MWCNT is significantly improved after the modification. The activity and selectivity for HDS are apparently higher for Co-Mo/MWCNT than for Co-Mo/Al2O3. For the modified samples, however, a slight and a significant decrease of activity and selectivity for HDS is observed for both Co-Mo/M-MWCNT-6 and Co-Mo/M-MWCNT-24 with a large proportion of byproduct found for the latter, indicating that significant modification is unfavorable for HDS.