The　authors　study　effectiveness　of　using　a　semiactive　variable　damper　to　control　seismically　excited　structures.　As　one　of　classical　examples　of　semiactive　control　system,　variable　dampers　are　widely　used　for　structural　control.　However,　recent　studies　show　variable　dampers　are　effective　in　reducing　seismic　responses　of　flexible　structures　while　it　can　be　used　to　control　responses　of　rigid　structures.　Performance　of　variable　dampers　for　vibration　control　of　structures　under　seismic　excitations　is　investigated　using　harmonic　wave　analysis.　The　results　show　that　the　performance　of　variable　dampers　in　reducing　seismic　response　of　structures　depends　on　the　ratio　of　the　disturbance　frequencies　to　the　natural　frequencies　of　structures,　rather　than　the　natural　frequencies　of　structures　only.　Therefore,　the　effectiveness　of　variable　dampers　is　associated　with　the　characteristics　of　the　ground　motion　that　excites　structures.　In　addition,　the　control　performance　of　a　variable　damper　is　compared　with　that　of　a　passive　damper.　It　is　found　that　in　most　cases　variable　dampers　are　not　more　effective　than　passive　dampers.