The effect of the ratio (λ) of reactant molecular diameter to catalyst pore diameter on the restrictive diffusion under a highly restrictive regime of hydrotreating reactions of heavy residue oils over CoMo/alumina-aluminum phosphate catalysts was investigated. Hydrotreating reactions of residual oils were carried out in a trickle bed reactor at 663 K and 7.6 MPa over a series of CoMo/alumina-aluminum phosphate catalysts. The values of the effective diffusivities and the relevant effectiveness factors for HDS and HDM reactions were determined by applying the Thiele relation to catalytic reactions. The effective diffusivities for the HDM reaction are always smaller than those for the HDS reaction. In addition, the effective diffusivity values decreased with increasing ratios of reactant molecular diameters to catalyst pore diameters for both HDS and HDM reactions, indicating a large restrictive diffusion effect. Two different regimes for restrictive diffusion were observed. The restrictive effect under reaction conditions is still severe for low values of λ. However, this effect does not seem prominent for high values of λ.