A nonlinear mathematical model is developed for studying the self-loosening behavior of preloaded countersunk threaded fasteners that are subjected to cyclic transverse loads. Torque components acting on the bolt are divided into pitch and resistance torque components; the net torque determines whether or not the bolt will rotate loose under the external excitation. The accumulation of the differential amount of loosening rotation increments is converted into the gradual loss of the bolt tension/clamp load. Although the loosening model incorporates several system variables, this study is focused on investigating the effect of thread and bearing friction coefficients on the loosening of fasteners with coarse and fine threads. Model prediction of the self-loosening behavior is experimentally validated.