It is known that the bending moment of pile foundations is generated not only by the inertial interaction due to superstructure, but also by the kinematic interaction due to large deformation of soft soil layers. Therefore, it is highly necessary to investigate the characteristic of kinematic pile bending. The present study focuses on the appropriate diameter of pile foundations affected by the kinematic interaction. In this research, the relation between the diameter of pile and its bending strain is evaluated based on theoretical approaches and the seismic deformation method. As a result, the inadequate diameter that maximizes the kinematic pile bending strain is found and its characteristics are investigated.