The present study develops a stiffness reduction based model to characterize fatigue damage in unidirectional 0° and 90° plies and (0°/90°) laminates of fiber-reinforced polymer (FRP) composites. The proposed damage model has been developed, based on (i) cracking mechanism and damage progress in matrix (Region I), matrix-fiber interface (Region II) and fiber (Region III) and (ii) corresponding stiffness reduction of unidirectional composite laminates as the number of cycles progresses. Fatigue damage of (0/90)°composite systems was obtained by integrating damage values and occurred in composite plies of 0 and 90° under fatigue cycles. The predicted fatigue damage results based on the proposed damage model were found to be in good agreement for both unidirectional and (0/90) FRP composites, as they were compared with experimental data for these materials, tested at various cyclic stress levels, stress ratios, and off-axis angles reported in the literature.