In this paper, the effect of autofrettage process parameters on the ultimate pressure that functionally graded spherical vessel can tolerate are investigated. FGM properties and autofrettage pressure are considered as important parameters. Assumptions are variation of properties of FGM in radial direction, the residual stress in the absence of Bauschinger effect with the operation of variable material property method for bilinear material and power law fraction distribution for FG vessel calculated. The stress distribution in loading phase is computed using projection method and rule of mixture for FGM and elastic solution for thick spherical vessel. In unloading phase the material behavior is assumed to be isotropic and residual stress computed by using superposition method for loading and unloading phase. For reloading phase the rules of linear mixture employed for estimating the ultimate strength of FGM. By assuming functionally graded material properties and autofrettage process as effective parameters on amount of pressure capacity of autofrettaged vessel, the effects of parameters discussed separately. The results illustrate considerable effect of volume fraction used in FGM (up to 35% compared to full metal case) and inhomogeneity exponent (up to 154% compared to homogeneous case) on amount of ultimate pressure which is mentioned in results of parametric analyzing.