This paper deals with a method to obtain frequencies of large structures by component modal tests. This method determines a coordinate that adds mass and stiffness to a tested component. Additional mass and stiffness are found by considering the effect of untested components. Modal test results significantly depend on a coordinate with additional mass and stiffness. In the proposed method, peak frequencies of two masses give key information to select the optimal coordinate. These masses are found by reducing mass matrices of a tested component and a structure to an arbitrary coordinate. This method selects the coordinate so that peak frequencies of two masses are separated from the target frequency of structures. A numerical example is given to show the effectiveness of the method.
