Bolt　loosening　is　one　of　the　most　common　failure　modes　in　bolted　joints.　Tightening　again　is　often　used　to　compensate　for　the　loss　of　clamping　force　in　many　critical　applications.　In　this　study,　the　effect　of　load　control　modes　and　lubrication　conditions　on　bolt　loosening　were　studied　by　experiments.　Then,　a　series　of　experiments　were　carried　out　on　the　tightening　again　after　bolt　loosening　under　transverse　load.　Finally,　in　order　to　study　the　cause　of　fatigue　fracture,　FEA　was　carried　out　by　using　an　accurate　three-dimensional　finite　element　model.　The　results　show　that　the　bolt　loosening　is　mainly　caused　by　relative　motion　between　the　two　clamped　plates.　With　oil　lubrication,　the　bolt　clamping　force　was　able　to　reach　the　magnitude　comparable　to　the　initial　preload　when　the　nut　is　released　to　a　specific　angle　(30　degrees　similar　to　60　degrees)　before　tightening　again.　Nevertheless,　the　times　of　tightening　again　should　be　limited　to　avoid　fatigue　fracture　of　the　bolt.