Theoretical　formulas　for　predicting　the　undrained　shear　strength　of　K-0　consolidated　soft　clays　under　the　stress　path　related　to　triaxial　and　plane　strain　tests　are　presented　within　the　framework　of　critical　state　soil　mechanics.　An　inclined　elliptical　yield　surface　is　adopted　to　take　account　of　the　initial　anisotropic　stress　state.　The　undrained　strength　is　determined　by　combining　the　undrained　stress　path　in　the　volumetric　stress-strain　space　and　the　initial　yield　surface　in　the　deviator-mean　stress　space.　The　derived　mathematical　expressions　are　functions　of　the　critical　state　frictional　angle,　the　plastic　volumetric　strain　ratio　and　the　overconsolidation　ratio,　which　can　be　simplified　into　the　solutions　for　isotropically　consolidated　clays　under　triaxial　tests　or　under　plane　strain　tests.　The　results　calculated　by　using　the　theoretical　formulas　obtained　in　this　paper　are　in　good　agreement　with　the　available　collected　test　results.　It　indicates　that　these　new　formulas　are　applicable　to　triaxial　and　plane　strain　tests　on　normally　and　lightly　to　moderately　overconsolidated　soft　clays.