In a recent construction industry, cable supported sσuctures such as a cable-stayed bridge or space stadium have been increasingly constructed according to rapidly upgrade their related technologies. Generally stay cables as a critical member need to be rearranged for being satisfied with design tension forces. In this purpose, a vibration method has been applied to estimate the tension forces exerted on existing stay cables. In this study, cable vibration tests were carried out to evaluate the cable tension forces comp와ing with theoretical and practical formulas. Using the measured frequencies ob않ined from free vibration and impulsive tests, an accuracy of the estimated tension forces is confirmed according to use the first single mode only or higher multiple modes.