The fatigue behavior of short glass-fiber reinforced nylon 66 under stress controlled fatigue tests was studied on the basis of the nonlinear dynamic viscoelasticity measurements. In order to analyze the effect of nonlinear viscoelasticity on the fatigue behavior, quantitative measurements of nonlinear viscoelasticity have been carried out based on Fourier analysis. It was found that the nonlinear viscoelastic behavior that was closely related to the irreversible structural change appeared markedly during fatigue process. The failure models in fatigue process were proposed based on the cross-section morphology under optical microscopic observation before final failure of the specimens. The fatigue behavior could be classified into the two failure mechanisms, depending on whether the fatigue test was carried out below or above glass transition temperature of the matrix nylon 66. The fatigue process proceeded with the following steps:(l) the damage started with void formation at fiber ends; (2) the microcracks propagated around the fiber ends (T less than or equal to T-g) or the microcracks propagated bring accompanied with debonding along the fiber sides and also, forming the crack walls(T > T-g); (3) the cracks propagated between the fiber ends(T greater than or equal to T-g) in a brittle manner, or the crack walls dominantly remained being connected by bridges(T > T-g) in a ductile manner; (4) the fast crack propagation occurred, after the crack reached to a critical size, and finally, the specimen failed.