Modal mass of a structure is essential for analyzing structural response under disturbance and for designing the mass type control devices used so as to enhance the serviceability of the structure Modal mass of real structure differs from that of analytic mathematical model due to the error inherent in analytical assumption and construction uncertainty. In this study, an identification method is proposed to calculate the modal mass of real structure based on H^∞ optimal model reduction. The modal mass is obtained from the relationship between observability and controllability matrices realized from system identIfication and typical 2-degree of freedom state space model. The proposed method is verified thorough three examples.