Structural　strain　modes　are　able　to　detect　changes　in　local　structural　performance,　but　errors　are　inevitably　intermixed　in　the　measured　data.　In　this　paper,　strain　modal　parameters　are　considered　as　random　variables,　and　their　uncertainty　is　analyzed　by　a　Bayesian　method　based　on　the　structural　frequency　response　function　(FRF).　The　estimates　of　strain　modal　parameters　with　maximal　posterior　probability　are　determined.　Several　independent　measurements　of　the　FRF　of　a　four-story　reinforced　concrete　frame　structural　model　were　performed　in　the　laboratory.　The　ability　to　identify　the　stiffness　change　in　a　concrete　column　using　the　strain　mode　was　verified.　It　is　shown　that　the　uncertainty　of　the　natural　frequency　is　very　small.　Compared　with　the　displacement　mode　shape,　the　variations　of　strain　mode　shapes　at　each　point　are　quite　different.　The　damping　ratios　are　more　affected　by　the　types　of　test　systems.　Except　for　the　case　where　a　high　order　strain　mode　does　not　identify　local　damage,　the　first　order　strain　mode　can　provide　an　exact　indication　of　the　damage　location.