Abstract: This paper presents an approximate analytical method far calculating the isothermal effectiveness factors (nth order reaction) and nonisothermal effectiveness factors (1st order reaction) in spherical and cylindrical porous catalyst pellets. By introducing an equivalent reaction temperature andwith rational assumption, certain analytical correlations can be obtained. Complications in calculations involved in heterogeneous catalytic reaction, can thus be rendered much simpler. These correlations can be readily used to predict the isothermal and nonisothermal catalyst effectiveness factors with good accuracy. It is pointed out that such a concept of equivalent reaction temperature with the proposed approximation method will be very useful in catalytic reactor design and catalyst analysis.