Al2O3 materials with different crystal forms were synthesized via the hydrothermal synthesis method using the low-cost raw material, and then the corresponding NiMo/Al2O3 catalysts were prepared by using Al2O3 materials with different crystal forms as the supports. The restrictive diffusion effects on the hydrodesulfurization reaction of different reactants with different molecular sizes over the NiMo/Al2O3 catalysts with different crystal forms were investigated systematically for the first time. NiMo/5-Al2O3 exhibited the highest values of the effective factor (eta) and effective diffusion coefficient (D-e), which could be ascribed to its proper pore diameter and relatively concentrated pore diameter distribution. The hindered magnitudes of diffusion decrease in the order of NiMo/delta-30 (2.23) < NiMo/theta-30 (2.83) < NiMo/gamma-30 (3.42), indicating that the restrictive diffusion effect of NiMo/delta-30 catalyst was weaker than those of the other two catalysts.