A　non-orthogonal　elastoplastic　constitutive　model　for　sand　with　dilatancy　is　presented　in　the　characteristic　stress　space.　Dilatancy　of　sand　is　represented　by　the　direction　of　plastic　flow,　which　can　be　directly　determined　by　applying　the　non-orthogonal　plastic　flow　rule　to　an　improved　elliptic　yield　function.　A　new　hardening　parameter　is　developed　to　describe　the　contractive　and　dilative　volume　change　during　the　shear　process,　which　is　coordinated　with　the　non-orthogonal　plastic　flow　direction.　The　combination　of　the　non-orthogonal　plastic　flow　rule　and　the　proposed　hardening　parameter　renders　the　proposed　model　with　the　ability　to　reasonably　describe　the　stress-strain　relationship　of　sand　with　dilatancy.　The　model　performance　is　evaluated　by　comparing　with　the　experimental　data　of　sand　under　triaxial　stress　conditions.