Abstract: A simple method of calculating the energy release rateof crack in functionally graded materials is proposed. The doublecantilever beam theory is used to derive the formulae of G_\rm I^\rmg. As an example for the use of this method three typical graded materialsare evaluated. The results show that the modulus gradient has a greatinfluence on the fracture energy. When the crack extends in thedirection where the modulus is getting smaller, the energy release rateis smaller than that in the opposite direction.