The rail clamp is the device to prevent that a container crane slips along rails due to the wind blast as well as to locate the crane in the set position through loading and unloading containers on a vessel in the quay. In order to design the rail clamp, we need to determine the proper wedge angle to minimize the sliding distance of a roller and the proper clamping angle of a locker to generate the initial clamping force of a jaw pad. The researches for the proper wedge angle have conducted. So in this study we look into the relationship between the clamping angle of a locker and the initial clamping force in the rail clamp with wedge angle of 10. The initial clamping force of the jaw pad was determined by the clamping angle of the locker. Therefore after we measured the locking force of a locker and the tensile stress of extension bar with respect to the clamping angle of a locker, such as 2.18, 3.05, 3.92, 4.80, we compared the results with the FEA results.