A　weight　function　method　based　on　strain　parameters　is　proposed　to　determine　the　critical　plane　in　low-cycle　fatigue　region　under　both　constant　and　variable　amplitude　tension-torsion　loadings.　The　critical　plane　is　defined　by　the　weighted　mean　maximum　absolute　shear　strain　plane.　Combined　with　the　critical　plane　determined　by　the　proposed　method,　strain-based　fatigue　life　prediction　models　and　Wang-Brown＇s　multiaxial　cycle　counting　method　are　employed　to　predict　the　fatigue　life.　The　experimental　critical　plane　orientation　and　fatigue　life　data　under　constant　and　variable　amplitude　tension-torsion　loadings　are　used　to　verify　the　proposed　method.　The　results　show　that　the　proposed　method　is　appropriate　to　determine　the　critical　plane　under　both　constant　and　variable　amplitude　loadings.