Abstract: According to the approximate analytical expression for effectiveness factorpreviously proposed by the authors, a great deal of calculations are made with power law type kinetic expressions and Langmuir-Hinshelwood kinetic expressions for three onedimensional catalyst pellets. The results show that our approximate solutions agree fairly well with the exact values obtained by numerical method in all circumstances investigated. The comparison with other approximate expressions in literature is made. As our expression is based on the analysis of the characteristics of the reaction system, the exactness is higher, the calculation is much easier, and above all, its scope of application is wider.