Considering　that　the　controller　feedback　gain　and　the　observer　gain　are　of　additive　norm-bounded　variations,　a　design　method　of　observer-based　H-infinity　output　feedback　controller　for　uncertain　Delta　operator　systems　is　proposed　in　this　paper.　A　sufficient　condition　of　such　controllers　is　presented　in　linear　matrix　inequality　(LMI)　forms.　A　numerical　example　is　then　given　to　illustrate　the　effectiveness　of　this　method,　that　is,　the　obtained　controller　guarantees　the　closed-loop　system　asymptotically　stable　and　the　expected　H-infinity　performance　even　if　the　controller　feedback　gain　and　the　observer　gain　are　varied.　©　2009　Editorial　Board　of　Control　Theory　and　Applications,　South　China　University　of　Technology　and　Springer-Verlag　GmbH.