This paper deals with fatigue strength and damage behavior of glass-particle-reinforced Nylon 66 composites. Fatigue tests are carried out on seven kinds of glass-particle-reinforced Nylon 66 composites in which a volume fraction of glass particles and interface treatment between particles and matrix are changed. The fatigue strength is low in the interface-untreated composite and is high in the interface-treated composite as compared with that of the Nylon 66. Variation of stiffness during fatigue is determined from the equilibrium stress-strain relations obtained by multi-step relaxation tests after given stress cycles, which eliminate the influence of viscous deformation of the Nylon 66 matrix. The stiffness reduces significantly at the early stage of stress cycles and then becomes constant in the Nylon 66 and both composites. In the fatigue tests under the applied stress for the same fatigue life, the stiffness reduction is more remarkable in both composites than in Nylon 66 because of the debonding damage. From the variation of the equilibrium stress-strain relations during fatigue, it is found that the viscous component of deformation gradually disappears with increasing stress cycles.