A cohesive zone model is proposed to analyse composite delamination propagation under high-cycle fatigue loading. The cohesive zone model formulation concept is discussed and the degradation of interlaminar mechanical properties is analysed from a physical point of view. Firstly, the damage evolution is evaluated according to the traction-separation law and it is demonstrated that if a linear elastic unloading/reloading curve is assumed, the softening function must also be linear. Secondly, issues regarding damage onset and fracture criteria in mixed-mode loading are critically addressed and commented. A new set of criteria is proposed, and the limitations of existing criteria are discussed. Furthermore, a new method to compute the strain energy release rate at any point within the element fatigue life cycle range is presented. The proposed scheme is based on the J-integral method evaluated at an estimated position of the crack-tip within the element, instead of the element integration points. Furthermore, a new fatigue damage evolution law is proposed to account for the unwanted quasi-static damage during the element fatigue degradation process. The model prediction capabilities were verified against experimental data available in the literature and theoretical solutions using a double cantilever beam configuration for mode I loading, four-point end-notched flexure configuration for mode II loading, and mixed-mode bending configuration for mixed-mode loading. The simulations were performed at both constant and variable amplitude loading.The numerical predictions obtained using the proposed model correlated very well with literature's experimental data.