Abstract Fatigue life prediction requires thorough understanding of the deformation and damage mechanisms and factors influencing them. The objective of this work is to identify the main micro-mechanisms which govern the fatigue behavior of a short glass fiber reinforced polyamide composite through a multi-scale experimental analysis. Tension-tension fatigue tests have been performed at different applied maximum stress and have been analyzed at both microscopic and macroscopic scale. Together with the progressive loss of stiffness, the temperature rise during cyclic loading has been measured using an infrared camera. Moreover, SEM observations have been performed on the fracture surfaces. The analysis of the results shows that the fatigue strength is a consequence of two principal mechanisms: matrix temperature rise and fiber-matrix interface damage. Micro-ductile areas have been observed around the fibers. A statistical quantitative analysis relates the size of these very deformed areas to the maximum applied stress. Moreover, it has been shown that damage can appear only after a stabilization of the temperature. Finally, the multi-scale analysis allows relating the stress threshold for each mechanism to the S-N curves. The effects of the frequency and of the mean stress have been also evaluated.