First of all, it is the most important to know the cause and effect of major failures to increase the reliability of servo motor. All reliability parameters can be achieved after analysis of them and experiments based on the analysis data. So, we firstly made out the failure analysis data of servo motor such as FMMA, FMECA, FTA, and 2-stage QFD considering the reference materials, field failure data, and the opinion of experts of servo motor. And the test items were decided which are needed to evaluate the performance characteristics of servo motor as analysis results of failure analysis data. And then, the major reliability parameters are deduced by experiments and MINITAB analysis. In addition, according to prove a major failure mode of servo motor by above analysis results, we produced FEA (Finite Element Analysis) model about shape of stator and rotor of servo motor to find out the reducing method of cogging torque. This paper presents the useful major reliability parameters, detailed failure mode analysis data, experimental results using 3 samples, and a cogging torque reducing method of servo motor, and also verified the validity of the deduced parameters and analysis data.