This paper concerns the effect of auxiliary fields and distance of contours from the crack tip on the accuracy of stress intensity factors of Functionally Graded Materials (FGMs), using the interaction integral method. In the first step, defining auxiliary fields of displacement, strain, and stress appropriately, the interaction integral is derived which is independent of derivatives of properties of the materials. Actual and auxiliary fields of displacement, strain and stress are used to compute the interaction integral. Actual fields are obtained by isoparametric finite element method, while auxiliary fields are constructed by use of the crack tip properties on the basis of William’s solution. These auxiliary fields are not appropriate, except near the crack tip. Therefore, different non-equilibrium and incompatibility formulations are used to consider the changes in non-homogeneous material. Considering the changes in FGMs as an exponential function, the results will be obtained from these formulations and are compared with others recorded in the literature. Furthermore, considering different contours, the effect of distance of contours from the crack tip on the stress intensity factors of FGMs is examined. The results confirm that the solutions using the incompatibility and constant constitutive tensor are more accurate. In contrast the non-equilibrium method is not proper for contours which are placed far away from the crack tip and presents less accuracy.