The　tension-torsion　fatigue　characteristics　were　investigated　under　proportional　and　non-proportional　loading　in　this　paper.　The　fatigue　cracks　on　the　surface　of　multiaxial　fatigue　specimens　were　observed　and　analyzed　by　a　scan　electron　microscope.　On　the　basis　of　the　investigation　on　the　Kindil-Brown-Miller　and　Fatemi-Socie＇s　critical　plane　approaches,　a　shear　strain　based　multiaxial　fatigue　damage　parameter　was　proposed　by　von　Mises　criterion　based　on　combining　the　maximum　shear　strain　and　the　normal　strain　excursion　between　adjacent　turning　points　of　the　maximum　shear　strain　on　the　critical　plane.　The　proposed　multiaxial　fatigue　damage　parameter　does　not　include　the　weight　constants.　According　to　the　proposed　multiaxial　fatigue　damage　parameter,　the　multiaxial　fatigue　life　prediction　model　was　established　with　the　Coffin-Manson　equation,　which　is　used　to　predict　the　multiaxial　fatigue　life　of　medium-carbon　steel.　The　results　showed　that　the　proposed　multiaxial　fatigue　damage　parameter　could　be　used　under　either　multiaxial　proportional　or　non-proportional　loading.　(C)　2006　Elsevier　Ltd.　All　rights　reserved.