The　buckling　behavior　of　defective　single-walled　carbon　nanotubes　(CNTs)　under　torsion　is　investigated　by　using　molecular　dynamics　simulations.　Various　kinds　of　defects　including　vacancy　defects　(monovacancy,　bivancancies　and　line)　and　topological　defects　such　as　Stone-Thrower-Wales　(STW)　are　considered.　The　effect　of　initial　defects　on　the　torsional　properties　is　closely　examined.　The　simulation　results　show　that　the　torsional　capacity　is　strongly　dependent　of　the　type　of　defects,　chirality　and　temperature.　The　reduction　in　the　torsional　capacity　is　greater　for　CNTs　with　vacancy　defects　than　CNTs　with　topological　defects.　Armchair　CNTs　have　higher　shear　modulus　and　critical　torques　and　are　less　sensitive　to　the　presence　of　defects　when　compared　to　their　zigzag　counterparts.