The creep behavior of a pressurized tank, which is assumed to be made of functionally graded materials, is studied in this paper. The elastic response under the internal and external pressures is first obtained when Young’s modulus obeys a power function along with the wall thickness. If the creep exponent remains constant and the creep coefficient varies with the radial coordinate, a closed-form solution can be derived for the time-dependent behavior of the spherical tank. The effects of material gradients on the creep stress and strain are investigated in detail. The results show that the stress level under the steady creep state is determined by the distribution of the creep properties. However, the magnitude of the creep strain is influenced by the elastic modulus distribution, as well as the creep property distribution inside the functionally graded materials. Compared with the finite element analysis results, the optimum time step value is also investigated. Some fundamental knowledge of the materials distribution is achieved to reduce the maximum creep stress/strain and to uniformize the stress level inside the functionally graded materials tank.