Due　to　its　strength　advantage,　high　strength　bolts　are　able　to　provide　larger　clamping　force.　However,　the　changing　behavior　of　friction　coefficient　during　repeated　tightening　will　adversely　affect　the　tightening　torque　clamping　force　relationship.　To　ensure　reliable　clamping　force　of　high　strength　bolts,　an　easy　implement　experimental　method　to　investigate　the　changing　behavior　of　friction　coefficient　is　put　forward　in　this　work.　The　constraint　torque　of　bolt　head,　as　well　as　the　tightening　torque　and　clamping　force,　was　measured　during　tightening.　Friction　coefficient　interval　of　bolt　underhead　is　obtained　according　to　the　preset　assumptions　and　experimental　results.　Based　on　the　mechanical　equilibrium　relationship　of　the　bolt　during　tightening,　thread　reaction　torque　can　be　obtained　to　calculate　the　friction　coefficient　of　threads　and　bearing　surface.　Three　cases　are　considered,　namely　without　lubricant,　lubrication　at　threads　and　nut　surface.　Statistical　analysis　was　carried　out　through　replicated　experiments　to　investigate　the　effect　of　clamping　force　on　the　changing　behavior　of　friction　coefficient.　Without　lubrication,　repeated　tightening　caused　the　increasing　of　friction　coefficient,　which　resulted　in　a　reduction　of　clamping　force.　When　using　a　lubricant,　it　can　protect　the　contact　surface,　and　the　obtained　clamping　force　during　repeated　tightening　was　more　stable.　Experimental　results　showed　that　the　proposed　method　can　be　used　to　investigate　the　changing　behavior　of　friction　coefficient　during　repeated　tightening.