Abstract: The time-dependent behavior and the affecting factors of preload relaxation in carbon/epoxy composite bolted joint have been studied, through a forced bending vibration test included various initial preloads and excitation frequencies. Analysis method and test method, based on modal parameters (resonance frequency and damping ratio), were utilized to characterize the dynamic property of bolted joints. It is observed that the loss degree of preload will increase with decreasing initial preload, and it will increase with the exciting frequency increasing, over a period of 10 h vibration fatigue process. Vibration fatigue damages would result in stiffness decay and damping increase in structural joints; the relaxation in composite bolted joints can be attributed to the conjunct mechanisms between viscoelastic behavior of materials and interface friction of which about 50% of relaxation is due to viscoelastic effect of composites.