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In this paper an exact solution of dynamic analysis of dam-reservoir interaction in time-domain is considered. The radiation condition of the infinite reservoir and flexibility of dam structure are included in formulation so the interaction problem is solved in closed-form. Problem is proposed by simplifying dam as a tapered beam. Dam thickness is assumed variable. The proposed method is used for analyzing the interactive behavior of dam-reservoir with different geometric properties, which is demonstrated by numerical examples when the system is subjected to Elcentro and Taft earthquakes. Dam displacement and hydrodynamic pressure is compared with empty and full reservoirs with those of various researchers and EAGD 8 (Fenves & Chopra) program and good performance of the method is demonstrated.