Abstract In this work, the flexural fatigue behavior of carbon fiber reinforced plastic (CFRP) composites with respect to stiffness degradation, fatigue life prediction and 3D damage mode with addition of nanoclay as matrix filler is investigated. Three-point static flexural tests were conducted to measure the elastic modulus and failure strength/deflection of the composites. Load-controlled flexural fatigue tests were performed at different stress levels; 0.9, 0.8, 0.75 and 0.7 times the static strength of the corresponding control and nanophased samples to determine the fatigue life and stiffness degradation, respectively. The fatigue life was reliably predicted by means of combined Weibull and Sigmoidal model. It was found that nanoclay added samples exhibited more than 327% improvement in the mean fatigue life and more than 352% improvement in the predicted fatigue life (Pf = 0.5), compared to the control samples. At 0.7 stress level, all the nanoclay added samples passed the ‘run-out’ fatigue criteria of one million cycles, whereas majority of the control samples failed much earlier. Conditioning (6 months under seawater at ambient temperatures) effect on the fatigue performance of the CFRP samples showed that both control and nanoparticles added samples demonstrated lower fatigue life in comparison with corresponding unconditioned samples.