The boundary element analysis is attempted to investigate the relaxation behavior of the bolt load in a single bolted joint fastening thermosetting polymers. More than 10 % relaxation is obtained after 4,000 minutes. The loss of bolt stress is more important because the stress in the polymer is much smaller. Such relaxation of the bolt stress can cause loosening for fasteners and loss of fastener effectiveness for bolts that rely on clamping force or preload to achieve needed joining forces. The experimental evaluation is required to clarify the effect of the loss of bolt load on function of the assembly.