Ground drilling technology for drilling has an environment where the major parts are prone to damage due to high stress, torque, and harsh operating conditions that can occur in the rotary power transfer structure. Research for preventing this damage is very important, as it can be coupled with the nature of drilling operations that take a long time in operation, which can lead to enormous cost and time consumption. Previous work investigated the cause of damage by analyzing the working environment and breakage of drilling holes for connecting rods, and a power transfer component of directional mud motors used in ground drilling systems. The material properties by heat treatment conditions for applied materials were analyzed. Based on prior work, we evaluated whether the stress concentration part shown in the analysis results matched the actual damage occurring point by conducting a structural analysis of the connecting rod, a damaged part, using the finite element analysis. We also analyzed how to reduce the stress concentration phenomenon that occurs during the mud motor operation by conducting part shape and design changes between the connecting rod and key parts.