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The modal mass is one of the most important dynamic properties of structure like a natural frequency and damping ratio. In this study, it is outlined that modal masses of a MDOF structure are directly extracted from the vibration data using the system identification. Modal masses are obtained from the relationship between observability matrix and controllability matrix defined in arbitrary and typical state space model, respectively, and assumption that the locations of input and output are identical. The mass matrix and eigen vectors of a structure are not required. Numerical analysis is performed to verified the method proposed in this study. The results show that the modal masses of each mode can be precisely estimated. It is also observed that the accuracy is the best in the first lower modes and decreased in the higher modes.