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The main reliability issue encountered during cylinder head development is the appearance of fatigue cracks during severe endurance tests. Some of these cracks can initiate in the coolant water jacket and lead to the complete part failure. They are mainly associated with the combination of combustion pressure alternate stresses and high cycle fatigue load in infinite life domain. However, Haigh or Dang Van analysis issued from 3D Finite Element Analysis reveals several issues: crack initiations are not correctly located and scatter of crack sizes are not predicted. To overcome these difficulties, a new fatigue model dedicated to cylinder heads in Aluminum - Silicon alloy has been developed in Renault powertrain division. The model is based on fatigue crack growth modelling in order to take into account various and combined load ratio in one hand, and typical characteristics of the microstructure of casting alloy in the other hand. This paper presents the theoretical basis of this model and the associated identification methodology for two different aluminum alloys.