Progressive　collapse　is　a　form　of　disproportionate　damage,　which　is　induced　by　an　initial　local　failure　that　propagates　throughout　a　large　portion　or　the　entirety　of　structural　systems.　The　initial　local　failure　may　occur　anywhere　in　a　structural　system,　which　consequently　exhibits　different　collapse　behaviors.　However,　the　progressive　collapse　resistance　is　an　inherent　property　of　the　entire　structural　system　regardless　of　any　specific　initial　local　failure.　This　article　proposes　a　collapse　probability　index　to　assess　the　progressive　collapse　resistance　of　reinforced　concrete　frame　structures.　Such　an　index　is　capable　of　measuring　all　typical　initial　local　failures　instead　of　any　specific　ones.　The　collapse　fragility　curve　is　further　established　based　on　the　collapse　probability　index,　which　varies　with　an　increase　in　the　nominal　gravity.　The　structural　redundant　resistance　to　progressive　collapse　can　be　assessed　by　the　fragility　curve.　Furthermore,　a　simplified　assessment　method　is　proposed　to　measure　the　collapse　probability　indices　feasibly　of　reinforced　concrete　frames　under　different　nominal　gravity　levels.　The　proposed　method　based　on　the　collapse　probability　index　can　comprehensively　evaluate　the　progressive　collapse　resistance　of　reinforced　concrete　frames　which　is　validated　through　analysis　of　a　single　four-story　and　a　group　of　eight-story　reinforced　concrete　frame　structures.