In　recent　years,　the　magnetorheological　(MR)　fluid　damper-based　semiactive　control　systems　have　received　considerable　attention　for　protecting　structures　against　natural　hazards　such　as　strong　earthquakes　and　high　winds.　In　this　paper,　an　improved　modal　controller　integrated　with　a　Kalman-filter　observer　is　proposed　for　vibration　control　of　spatial　structures.　The　proposed　modal　controller　can　reduce　the　dimension　of　the　control　equation　and　thus　can　relieve　the　calculation　burden　greatly.　In　the　proposed　control　system,　a　Kalman-filter　observer,　which　estimates　the　full　controlled　modal　states　from　local　accelerometer　outputs,　is　designed　for　rendering　the　controller　to　be　more　applicable　to　real　spatial　structures　with　a　large　number　of　degrees　of　freedom　(DOFs).　A　numerical　example　of　a　lattice　roof　structure　installed　with　MR　dampers　is　presented.　The　effectiveness　of　the　controller　is　evaluated　under　both　Tianjin　and　El　Centro　earthquake　excitations.　The　simulation　results　demonstrate　not　only　its　effectiveness　of　reducing　vibration　of　spatial　structure,　but　also　its　adaptability　for　versatile　loading　conditions.