Morphological features of voids were characterized for T300/924 12-ply and 16-ply composite laminates at different porosity levels through the implementation of a digital microscopy (DM) image analysis technique. The composite laminates were fabricated through compression molding. Compression pressures of 0.1MPa, 0.3MPa, and 0.5MPa were selected to obtain composite plaques at different porosity levels. Tension-tension fatigue tests at load ratio R=0.1 for composite laminates at different void levels were conducted, and the dynamic stiffness degradation during the tests was monitored. Fatigue mechanisms were then discussed based on scanning electron microscope (SEM) images of the fatigue fracture surfaces. The test results showed that the presence of voids in the matrix has detrimental effects on the fatigue resistance of the material, depending on the applied load level. A fatigue model was also established in this paper by introducing the void content into the conventional residual stiffness approach. The residual stiffness model can evaluate the void effects on the fatigue behaviors of CFRP unidirectional laminates. The dynamic stiffness degradation of composite laminates at different porosity levels and different load levels was predicted in this study. The prediction results show a good correlation with the test data.