Abstract: The influence and mechanism of fitting modes including clearance and interference on joint stiffness were researched as far as single-bolt, single-lap composite bolted joint was concerned. The validation of the adopted finite element method was confirmed by comparing with the results of tests. The results illustrate that for single-bolt, single-lap composite bolted joint, clearance fit reduces joint stiffness and the joint stiffness decreases almost linearly as clearance increases, clearance fit of 3% bolt diameter decreases joint stiffness by 16%-17%. For joint without hole delamination damage during assembling process, interference fit of 0.5% bolt diameter increases joint stiffness by about 15%. With different fitting modes, the contact stress peak direction and the uneven degree of contact stress distribution through thickness are distinct, which influence the joint stiffness.