The　rock　mass　classification　methods　developed　to　evaluate　rock　mass　stability　and　tunnel　support　design　for　blast　and　drill　tunnels　are　not　suitable　to　guide　tunneling　by　tunnel　boring　machine　(TBM),　such　as　selection　of　TBM　types,　determination　of　construction　scheme　and　so　on.　By　comprehensively　considering　the　rock　mass　boreability　and　stability,　a　modified　rock　mass　classification　system　for　TBM　tunnels　is　proposed　on　the　basis　of　the　hydropower　rock　mass　classification　(HC)　method　of　China.　The　input　parameters　are　five　rock　mass　parameters　as　the　same　as　the　HC　method.　In　addition,　the　tunnel　diameter　is　needed　to　estimate　the　TBM　advance　rate.　The　structure　and　parameter　determination　of　the　classification　system　are　explained　and　statistically　analyzed　in　details.　This　system　can　be　applied　to　evaluate　the　feasibility　of　TBM　construction　and　serve　as　the　basis　for　selection　of　TBM　types　and　design　parameters　in　the　pre-construction　phase.　Furthermore,　it　can　also　be　applied　to　estimate　TBM　performance　and　guide　tunnel　support　for　a　given　ground　condition　in　the　design　phase,　and　to　optimize　TBM　operational　parameters　during　the　construction　phase　as　well.　An　example　is　presented　to　illustrate　the　application　of　the　proposed　system.　The　limitations　of　the　proposed　system　and　further　studies　are　also　discussed.