In　order　to　improve　the　current　urban　traffic　assignment　models,　a　multimodal　stochastic　user　equilibrium　model　based　on　nested　Logit　was　proposed,　considering　the　behaviors　of　mode　choice　and　route　choice.　Firstly,　in　a　multimodal　traffic　network,　a　nested　Logit　model　was　proposed　to　describe　the　behavior　of　the　combined　traffic　mode　and　route　choice,　according　to　the　two　steps　of　a　traveler＇s　travel　decision　(he　or　she　chooses　an　appropriate　traffic　mode　and　then　chooses　a　route　in　the　traffic　network).　In　the　proposed　nested　Logit　model,　the　utility　and　traffic　flow　distribution　were　mutually　restricted,　the　utility　of　the　travel　was　influenced　by　the　network　traffic　flow,　and　the　traffic　flow　was　determined　by　the　utility,　thus　leading　to　equilibrium.　Therefore,　a　mathematical　programming　model　for　finding　the　equilibrium　solution　was　developed　in　this　study.　It　was　proved　that　the　optimal　solution　of　the　mathematical　programming　model　and　the　flow　solution　of　the　nested　Logit　model　were　equivalent.　It　was　further　proved　that　both　the　objective　function　of　the　equivalent　mathematical　programming　model　and　the　constraint　set　were　convex.　Therefore,　the　equivalent　mathematical　programming　model　was　a　convex　programming　problem,　and　the　solution　of　the　model　was　unique.　Subsequently,　the　step　of　improved　the　direction　search　algorithm　was　given　to　solve　the　proposed　multimodal　stochastic　user　equilibrium　assignment　model　based　on　nested　Logit.　Finally,　a　case　study　of　a　grid　network　with　two　modes,　namely　bus　and　car,　was　carried　out　to　analyze　the　impact　of　bus　fare,　traveler＇s　time　value,　and　the　interlayer　ratio　parameter　of　the　nested　Logit　model　on　traffic　assignment　results.　The　results　show　that　the　changes　in　bus　fare　have　little　effect　on　the　traffic　assignment　results,　and　that　bus　demand　decreases　with　increase　in　travelers＇　time　value.　When　the　travelers＇　time　value　is　low,　the　bus　demand　increases　with　increase　in　the　interlayer　ratio　parameter　of　the　nested　Logit　model,　whereas　when　the　travelers＇　time　value　is　high,　the　bus　demand　decreases　with　increase　in　the　interlayer　ratio　parameter　of　the　nested　Logit　model.　2　tabs,　7　figs,　20　refs.　©2018,　Editorial　Department　of　Journal　of　Chang＇an　University　(Natural　Science　Edition).　All　right　reserved.