In　view　of　that　a　machine　is　unavailable　due　to　breakdowns　or　preventive　maintenances　in　the　practical　manufacturing　environment,　the　two-machine　flow　shop　scheduling　problem　with　an　unavailable　interval　on　any　machine　is　studied.　The　optimality　condition　of　solving　the　problem　by　the　Johnson　algorithm　is　presented.　The　worst-case　performance　ratio　of　the　Johnson　algorithm　as　a　heuristic　is　analyzed.　The　results　provide　the　theoretical　bases　for　exploring　the　optimal　algorithm　of　the　two-machine　flow　shop　scheduling　problem　with　an　unavailable　interval.　©,　2015,　Nanjing　University　of　Science　and　Technology.　All　right　reserved.