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The paper presents a simple pyramidal model of a real intergranular crack front enabling the analytical calculation of the effective stress intensity factor. This analytical approach yields values well comparable with the data obtained by a boundary element numerical procedure applied to real-like intergranular crack fronts. It enables to separate the geometrical shielding level from the measured fracture toughness values in order to obtain the real averaged grain boundary fracture energy. In this way, some apparently paradox phenomena can be quantitatively elucidated, e.g. the increase in fracture toughness with increasing solution annealing temperature in ultra-high strength low alloy steels.