Fibre reinforced polymer composites are frequently used in the design of tidal turbine blades where cyclic stresses brought on by the velocity fluctuations of tidal currents limit the in-service life of the blade. This paper presents a fatigue life prediction methodology for composite tidal turbine blades based on combined hydrodynamic and finite element structural models with the view to characterize the fatigue properties and failure modes of candidate composite material manufactured by vacuum assisted resin transfer moulding process. A hydrodynamic analysis is presented of the tidal flows over a typical tidal turbine blade. Using stream tube method, the temporal variations of tidal flows including the effect of support structure and waves provide the cyclic loading of the blades. The fatigue data for the tested material under realistic marine conditions is then used as input into the fatigue model which allows the prediction of the fatigue life. Incorporating fatigue failure modes with the finite element stress analysis of tidal turbine blades, showed that environmentally affected fatigue failure modes such as delamination is associated with blade root area when the blade is subjected to the cyclic tidal currents. Case study results for two commercial scale tidal turbine blades with energy output of 1.5 and 0.35 MW are shown to establish the methodology. The tidal turbine blade design methodology presented here can be used to assess the life expectancy of composite blade and tidal device energy production.