Abstract: Isothermal effectiveness factors for slab, cylinder and sphere shaped catalysts with uniform or nonuniform intrinsic activity profiles have been investigated. In the case of zero-, first- and second-order kinetics, the effectiveness factors of pellets with increasing activity towards the pellet surface are larger than that of uniform active catalyst, and they are proportional to the square root of the activity at the pellet surface with significant diffusion effect. The effectiveness factor-Thiele modulus curves which are valid for both uniform and nonuniform catalysts have been obtained with the Thiele modulus modified by equivalent thickness of effective layer of the catalyst. Thus, the effectiveness factor for nonuniform active catalyst could be predicted with a maximun deviation of 5% in the case of significant or insignificant diffusion effect, but 10% in general.