Inclusion　of　flexible　fibers　such　as　polypropylene　and　polyester　is　an　effective　method　for　soil　improvement,　as　it　significantly　enhances　the　soil　strength　and　ductility.　A　proper　constitutive　model　is　essential　for　assessing　the　stability　and　serviceability　of　fiber-reinforced　slopes/foundations.　A　new　method　for　constitutive　modeling　of　fiber-reinforced　sand　(FRS)　is　proposed.　It　assumes　that　the　strain　of　FRS　is　dependent　on　the　deformation　of　the　sand　skeleton　only,　while　the　effective　skeleton　stress　and　effective　skeleton　void　ratio,　which　should　be　used　in　describing　the　dilatancy,　plastic　hardening　and　elastic　stiffness　of　FRS,　are　affected　by　fiber　inclusion.　The　effective　skeleton　stress　is　dependent　on　the　shear　strain　level,　and　the　effective　skeleton　void　ratio　is　affected　by　the　fiber　content　and　sample　preparation　method.　A　critical　state　FRS　model　in　the　triaxial　stress　space　is　proposed　using　the　concept　of　effective　skeleton　stress　and　void　ratio.　Four　parameters　are　introduced　to　characterize　the　effect　of　fiber　inclusion　on　the　mechanical　behavior　of　sand,　all　of　which　can　be　easily　determined　based　on　triaxial　test　data　on　FRS,　without　measuring　the　stress-strain　relationship　of　individual　fibers.　The　model　is　validated　by　triaxial　compression　test　results　on　four　fiber-reinforced　sands　under　loading　conditions　with　various　confining　pressures,　densities　and　stress　paths.　Potential　improvement　in　the　model　for　incorporating　fiber　orientation　anisotropy　is　discussed.