Abstract: The author calculated the root stress of involute spur gear pair under the condition of different gear rim thicknesses by using KISSsoft and the finite element analysis, which of the results were different widely, and rim stress increased with the decreasing of rim thickness.But the calculation of root stress by using KISSsoft without the rim thickness factor, the transverse load factor and the face load factor was close to the value of FEA.The calculation results reveal that the rim thickness factor of KISSsoft is the response of the rim stress of FEA,and the gear rim failure of the latter appears through the excessive stress of the former, in order to remind the designer to optimize the gear rim and improve gear strength.