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 in the logarithmic plot of reaction rate and reaction probability. However, the low-slope one did not appear when the reaction occurred within the uniform pellet. Moreover, it was certified that the fractal structure not only generated a new asymptote, but also reduced diffusion resistance of reactants and products.