A　jointless　bridge　could　fundamentally　eliminate　vulnerable　deck　joints,　thereby　meeting　the　need　for　sustainable　development　of　bridges,　especially　for　an　expressway　with　high-speed　traffic.　In　this　paper,　one　jointless　bridge　(deck-extension　bridge)　with　a　small　box　girder　in　an　expressway　was　chosen　as　a　case　study　to　examine　the　structural　design,　construction　and　field　test.　The　field　tests　of　the　bridge　indicated　that　the　designed　and　constructed　structures　can　satisfy　the　requirement　for　service　performance　of　the　deck-extension　bridge.　Some　key　technologies,　such　as　the　position　of　longitudinal　reinforcements　in　the　superstructure-approach　slab　connections　and　the　arrangement　of　the　sliding　material　layers,　were　introduced.　The　longitudinal　thermal　movement　of　the　superstructure　in　the　deck-extension　bridge　with　a　small　box　girder　could　be　predicted　accurately　by　using　the　average　temperature　of　the　cross　section　of　a　small　box　girder.　The　finite　element　model,　built　by　using　the　MIDAS　program,　was　used　to　analyze　the　temperature　distribution　on　the　cross　section　of　a　small　box　girder,　the　accuracy　of　which　could　be　verified　by　comparing　with　the　measured　values.　The　maximum　longitudinal　thermal　movement　of　the　superstructure　in　deck-extension　bridges　with　a　small　box　girder　under　historically　extreme　temperature　conditions　was　predicted.　©　2018　The　Authors