Abstract: The modal superposition method is often used to obtain mode shape derivatives. However, it can not give an accurate solution and the errors may become significant, when more high-frequency modes are truncated. This paper deals with the contribution due to the unavailable high-frequency modes to mode shape derivatives, proves that it can be expressed by the available low-frequency modes and the system matrixes, and presents a numerical algorithm. Numerical examples show that the method is correct and can imp...