Energy dissipation systems have been broadly used in structures during the recent decades in order to reduce earthquake and wind forces as well as reduction of structural lateral drifts within the code limits. Viscous damper is considered as one of the energy dissipation systems which are classified as velocity-dependent dampers among passive control systems and have been paid attention and their further detailed properties taken into account by many researchers. Viscous damper consists of a piston with some orifices inside the cylinder which contains highly viscous fluid. Energy dissipation of this damper is through pushing viscous fluid out of the orifices.These are two types of these dampers: linear and nonlinear from which linear type with velocity power of one is more practical. The structural damping force is usually set according to the procedure described in FEMA273 and are optimized by three controlled modes of displacement, velocity and acceleration.This study examines the effect of adding viscous dampers on seismic behavior of steel moment frames. For this purpose, three steel moment frames of 1, 3 and 6-story all in 3 bays, with viscous dampers having power factor of 1, 0.8 and 0.6 are selected. These sample structures are subjected to the nonlinear time history analysis under the El Centro, Kobe and Northridge earthquakes, and their response including displacement, acceleration and base shear is compared in two cases of with and without viscous dampers.Finally, regarding nonlinear time history analysis results based on the structural behavior in three modes controlled by displacement, velocity and acceleration, proper damping forces are specified as 94.7, 240.1 and 557.1 kN respectively for one, three and six story structures based on maximum acceleration and base share obtained from FEMA273.