Due　to　the　limited　bandwidth,　the　measurements　in　networked　control　systems　are　often　subject　to　random　communication　delays,　which　will　have　negative　effects　on　the　system　performance.　This　paper　deals　with　the　problem　of　fault　detection　for　networked　control　systems　with　random　communication　delays.　A　general　observer-based　fault　detection　filter　is　designed　and　applied　into　the　residual　generation　system.　Then　the　design　of　fault　detection　filter　is　formulated　as　an　H-infinity　filtering　problem　in　terms　of　H-infinity　theory.　A　sufficient　condition　for　the　existence　of　the　filter　is　established　by　means　of　linear　matrix　inequalities　(LMI)　at　last.　A　numerical　example　shows　the　validity　of　the　proposed　method