Coupling　failure　modes　of　strength　and　stiffness　in　structural　system　are　expressed　as　parallel　system.　The　step　by　step　equivalent　linearization　method　is　adopted　to　calculate　the　relativity　between　strength　failure　mode　and　stiffness　failure　mode　and　to　the　failure　probability　of　their　intersection　at　the　each　failure　state　of　strength　failure　mode.　By　example,　failure　probability　of　semi-rigid　steel　frame　is　analyzed　at　the　each　failure　state　and　compared　with　that　of　its　corresponding　rigid　frame.　The　results　show　that　semi-rigid　joints　increase　coupling　failure　probability　of　frame　strength　and　stiffness　and　make　it　main　failure　mode.　Semi-rigid　steel　frame　considering　coupling　failure　modes　of　strength　and　stiffness　is　suitable　to　real　failure　case.