Abstract: Two key problems in structural durability design for mechanically fastened carbon fiber reinforced epoxy composite joints:temperature-time dependent behavior for viscoelastic preload relaxation and prediction method for its long time performance were investigated. Preload relaxation prediction model was developed on basis of the creep total-strain theory. 36 h constant temperature durability tests show that the relaxation rate of preload for joints is faster with the greater initial preload and higher temperature; relaxation rate of composite joints is much greater than that of metallic joints; preload relaxation mainly behaves as the material creep process. Compared to the short-term test results, it shows that the model can well predict the relaxation for different temperatures, preloads and joint materials, which also provides a basis for extrapolation method of determining test data.