The　internal　nozzle　flow　has　great　influence　on　fuel　injection　and　spray.　This　work　investigates　the　impacts　of　the　injection　back　pressure　on　the　nozzle　inner　cavitation　developing,　especially　the　flow　characteristic　during　choking　process.　Based　on　the　theoretical　analysis,　a　three-dimension　numerical　model　is　developed　to　investigate　the　details　of　the　inner　nozzle　cavitation　flow.　The　numerical　model　is　verified　by　experiments　on　a　diesel　fuel　system　test　rig　under　different　boundary　pressures,　including　varied　injection　pressure　and　back　pressure.　The　investigation　shows　that　for　a　given　injection　pressure,　cavitation　occurs　with　the　drop　in　the　back　pressure.　The　incipient　bubbles　appear　just　at　the　upper　corner　of　nozzle　inlet　hole,　and　the　cavitation　area　continues　to　expand　until　it　reaches　to　the　nozzle　outlet.　The　nozzle　inner　cavitation　is　divided　into　three　periods:　no　cavitation　period;　local　cavitation　developing　period　and　super　cavitation　period.　Before　the　super　cavitation,　as　the　back　pressure　drops,　bubbles　at　the　entrance　of　the　nozzle　hole　cover　more　area,　but　the　average　liquid　velocity　increases　and　as　a　result　the　mass　flow　increases.　Once　the　liquid　velocity　reaches　to　its　maximum,　the　cavitation　of　the　nozzle　inlet　hole　maintains　stable,　so　does　the　vapor-liquid　mixture　cross　section,　and　the　effective　cross　section　of　the　liquid　phase　is　minimum;　by　now　the　super　cavitation　forms.　Under　the　condition　of　the　super　cavitation,　the　nozzle　inner　flow　becomes　choking　and　the　back　pressure　has　little　influence　on　the　mass　flow,　but　the　discharge　coefficient　declines　as　the　back　pressure　decreases.　During　the　choking　flow　period,　when　the　back　pressure　declines,　along　the　nozzle　hole　central　axis　from　the　inlet　to　the　outlet,　the　effective　flow　cross　section　maintains　stable　in　sequence　and　the　liquid　velocity　also　reaches　its　maximum　in　order.　Moreover,　at　the　outlet　of　the　nozzle　hole,　super　cavitation　induces　the　increase　of　the　turbulence　kinetic　energy　and　the　outlet　velocity.　This　increased　outlet　velocity　makes　up　for　the　loss　of　the　contracting　effective　flow　passage　due　to　super　cavitation,　thus　the　total　mass　flow　maintains　the　same　value　after　choke　even　the　back　pressure　decreases.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.