To　improve　the　load　consistency　in　heavy　truck　assembly,　a　high-precision　fastening　instrument　is　developed　to　simulate　bolted　assemblies　with　various　materials　and　sizes.　This　instrument　enables　to　control　the　tightening　process　with　torque,　torque-turn,　or　torque-to-yield　strategy.　The　tightening　torque　and　the　preloading　force　can　be　measured　in　real　time.　Adopting　the　design　of　experiment　(DOE)　method,　a　tightening　tests　program,　characterized　by　six　variables　with　two　levels　each,　is　designed　to　investigate　the　influence　of　six　factors　on　the　torque-preload　relationship　(nut　factor)　in　bolted　joints.　These　factors　include　joint　materials,　fastener　class,　gasket　grade,　lubrication,　surface　roughness,　tightening　speed　and　repeated　tightening.　The　effects　of　these　factors　on　the　nut　factor　and　their　interactions　are　evaluated　with　statistic　technique.　The　results　show　that　the　surface　roughness　and　the　joint　materials　are　the　major　factors　affecting　the　nut　factor;　the　interactions　between　the　joint　materials　and　the　gasket　grade　and　the　interactions　between　the　joint　materials　and　the　lubrication　also　exert　considerable　influence.　The　nut　factor　decreases　rapidly　with　the　tightening　times,　and　it　remains　stable　after　four　tightenings.　©　2015,　Editorial　Office　of　Journal　of　Xi＇an　Jiaotong　University.　All　right　reserved.