A　damage　identification　index　should　be　sensitive　to　the　local　structural　damage　and　its　computational　response　should　cater　for　the　requirement　of　on-line　structural　monitoring　system.　By　adopting　natural　excitation　technique　and　mathematical　statistics　theory,　a　damage　index　is　established,　which　is　based　on　the　strain　measurement　of　structures　under　environmental　excitation.　Numerical　simulation　is　carried　out,　it　reveals　that　the　damage　index　based　on　strain　is　more　sensitive　than　that　based　on　displacement　or　natural　frequencies.　The　damage　index　can　detect　the　occurrence　and　find　out　the　approximate　location　of　structural　damages.