The Monte Carlo method was employed to simulate diffusion and reaction processes within three-dimensional porous catalyst pellets. The porous pellets used were represented by a Menger sponge and a uniform-pore structure respectively. Results obtained from the fractal pellet showed an intermediate low-slope asymptote inthe logarithmic plot of reaction rate and reaction probability. However, the low-slope one did not appear when thereaction occurred within the uniform pellet. Moreover, it was certified that the fractal structure not only generateda new asymptote, but also reduced diffusion resistance of reactants and products.