Strain wave gears are widely used as reducers in robots, including collaborative and industrial robots. As a key component, they play a crucial role in determining overall robot performance. To enhance their effectiveness, various studies have focused on directly measuring the performance of assemblies or predicting the performance of individual components through analysis. However, there is a notable lack of research that experimentally measures and compares the physical properties of the circular spline, flexspline, and wave generator—the primary elements of strain wave gears. In this paper, we developed equipment to measure the radial stiffness of the flexspline, one of the key components, and validated its reliability through preliminary experiments. Furthermore, we measured and compared the radial stiffness of flexsplines produced by three different manufacturers. These findings are expected to provide valuable insights for improving the performance of strain wave gears and advancing robotics technology.