Unsupported MoS2 particles, which were produced in the preparation of MoS2/Al2O3 using a sonochemical method, were successfully separated from the prepared sample catalyst by adding oleylamine as an agent for dispersing the unsupported particles. The fraction of the unsupported MoS2, which was estimated based on Mo balance, varied between 0.03 and 0.4, independent of the Mo loading levels investigated (6-54 wt% of Mo). The activity of the unsupported MoS2 for the hydrodesulfurization of dibenzothiophene was nearly the same as that of the Al2O3-supported MoS2, indicating that the activity of the prepared catalyst was not affected by the presence of the unsupported MoS2 particles.