The　normal　static　stiffness　plays　an　important　role　in　the　whole　connection　of　bolted　joints.　Both　the　design　and　the　connection　quality　are　depended　on　the　normal　static　stiffness.　But　the　nonlinear　characteristics　of　its　normal　static　stiffness　is　not　well　explained.　An　experimental　test　is　developed　herein　to　clarify　the　influence　of　the　jointed　surfaces.　And　the　nonlinear　effect　of　jointed　surfaces　of　bolted　members　has　been　separated　from　the　total　stiffness.　The　nonlinear　relation　between　the　extensile　force　and　the　joints＇　stiffness　in　the　jointed　surfaces　under　different　preloads　has　been　extracted　through　the　experiment.