To　evaluate　the　effect　of　misalignment　between　the　combustor　exit　and　the　first　nozzle　guide　vane　endwall　on　the　endwall　thermal　load　distribution　and　operation　life,　the　influences　of　upstream　backward-facing　step　geometry　on　the　endwall　flow　field　and　heat　transfer　characteristics　were　numerically　investigated　for　a　transonic　turbine　cascade　using　CFD　solver　ANSYS　Fluent　v.　15.　The　accuracy　of　the　numerical　method　based　on　Reynolds　Stress　Model　(RSM)　was　demonstrated　with　the　experimental　data　of　endwall　heat　transfer　coefficients　for　two　upstream　step　heights　(0,　6.78　mm)　at　the　typical　design　condition　with　exit　Mach　number　of　0.85　and　inlet　turbulence　intensity　of　16%.　Numerical　simulations　were　conducted　for　six　different　upstream　backward-facing　step　heights　(0,　1.5,　3,　5,　6.78,　10　mm)　to　reveal　the　relationship　between　the　thermal　load　distribution　and　the　secondary　flow　structure　near　the　endwall,　and　explain　the　effects　of　upstream　step　geometry　on　the　formation　and　development　of　vortices　behind　the　step　as　well　as　the　aerodynamic　loss.　Several　conclusions　can　be　drawn　as　follows:　the　upstream　endwall　misalignment　will　produce　a　series　of　significant　complex　vortices　including　reattached　vortices,　cavity　vortices　and　auxiliary　vortices.　An　obvious　high　thermal　load　region　is　formed　upstream　of　the　vane　leading　edge　where　the　incoming　boundary　layer　fluid　sweeps　the　step　and　reattaches　the　vane　endwall.　As　the　step　height　increases,　this　high　thermal　load　region　is　pushed　downstream　and　more　into　the　vane　passage,　and　shows　an　obvious　increase　in　the　controlling　area　and　heat　transfer　level　(by　15%-160%).　In　addition,　the　shape　of　this　high　thermal　load　region　is　changing　from　the　＇bar＇　in　pitch　wise　to　＇C＇　with　both　ends　extending　to　the　vane　passage.　The　total　pressure　loss　appears　to　have　a　slight　increase　(by　0.17%-0.　45%)　due　to　the　backward-facing　upstream　step　geometry,　and　the　maximum　loss　occurs　when　the　step　height　is　3　mm.　©　2018,　Editorial　Office　of　Journal　of　Xi＇an　Jiaotong　University.　All　right　reserved.