A　new　method　is　presented　to　develop　the　existing　elastic-plastic　constitutive　model　into　an　elastic-plastic-viscous　one　for　clays.　The　actual　loading　process　is　divided　into　an　instant　process　and　a　delayed　process　denoting　the　elastic-plastic　strain　and　viscous　strain,　respectively.　The　elastic-plastic　strain　is　determined　by　either　an　elastic-plastic　model　for　overconsolidated　clays　or　an　improved　model　based　on　the　elastic-plastic　model　for　normally　consolidated　clays.　In　order　to　calculate　viscous　strain,　a　reference　state　line　is　defined　based　on　the　actual　loading　path.　Combining　the　reference　state　line,　an　existing　elastic-plastic　model　can　be　conveniently　developed　into　an　elastic-plastic-viscous　model.　Furthermore,　using　the　proposed　method,　the　modified　cam　clay　model　is　extended　into　an　elastic-plastic-viscous　model.　Comparisons　with　test　results　demonstrate　that　the　extended　model　can　capture　the　main　time-dependent　behaviours　of　clays,　including　creep,　stress　relaxation　and　strain　rate　effects.