The aim of the research is to assess the influence of the method accounting damping on the mass damper efficiency and selecting their optimal parameters for seismic impact. Particular attention is paid to the behavior of mass dampers of large mass. The steady-state harmonic oscillations of a damped system with a mass damper are considered. The design model is represented by two masses connected by springs. The mass damper optimal parameters are determined by minimizing the peak displacements on the amplitude-frequency characteristic. Three methods of damping accounting are considered: hysteresis in the structure and damper spring, equivalent viscous in the structure and in the damper spring, as well as hysteresis in the structure and in the damper spring and additional viscous in the damper spring. It has been established that when the mass of the damper is less than 10% of the mass of the protected structure, the method of accounting for energy losses in the structure and in the mass damper spring is not essential. With a damping mass of more than 50% of the structure weight, the method of damping accounting becomes fundamental important. It is noted that for highly damped systems, the efficiency of mass damper is noticeably reduced. Apparently, the ambiguity and inconsistency of recommendations on the damping selection for mass dampers available in the literature is associated with a significant influence of the way system damping is taken into account. It is recommended to consider the damping in the damper spring and in the structure separately. In a structure, damping does not usually exceed 15% of the critical one and the way it is taken into account is not important, but special dampers are used in the damper spring and their consideration must correspond to the actual damper loading diagram.