Shear　lag　and　negative　shear　lag　effect　in　cantilever　box　girders　are　analyzed　through　a　variation　approach　and　finite　element　techniques.　The　complex　distribution　of　bending　stresses　under　symmetrical　loading　on　the　flanges　of　cellular　sections　are　also　presented.　In　this　paper,　the　supposition　of　the　spanwise　displacement　on　flanges　follows　a　quartic　parabolic　variation.　However,　expressions　are　derived　to　determine　the　region　of　negative　shear　lag　effect　with　the　interrelation　of　span/width　parameters　involved.　The　theoretical　results　thus　obtained　are　compared　with　a　plexiglass　model　test.　Finally,　conclusions　are　drawn　with　regard　to　further　study　and　research.　©　ASCE.