Abstract: The viscoelastic damping control of steady-state harmonic response of stiffened plate is studied by using modal superposition method. Three cases of stiffened plate with damping treatment are considered and compared: only with free damping layer and only with viscoelastic beam and with both damping treatments. The influences of material loss factor, thickness of free damping layer and section of viscoelastic beam are analyzed. The relational curves of structural response amplitude and load frequency show that the viscoelastic material is effective for control of structural resonant response.