Abstarct: In present study, the governing equations of pressurized thick cylindrical shells are derived in static equilibrium condition, with nonlinear strain-displacement (kinematic) relations (Green-Lagrange strain tensor), and baxsed on the First-order Shear Deformation Theory (FSDT). The problem is considered to be full axisymmetric and thickness of cylinder is constant. Shell is made up of heterogeneous and isotropic material, with variable modulus of elasticity through the thickness according to the power law, and obeys linear elastic constitutive equations. As a common assumption, Poisson’s ration is supposed to be constant throughout the shell. The cylinder is clamped at both lengthwise bounds and undergoes uniform, time-independent, conservative (non-follower) internal and external pressures. The governing equations form a coupled system of nonlinear differential equations of second order, with constant coefficients. These equations are derived using virtual work principle and solved utilizing perturbations theory (matched asymptotic expansion method).The analytical solution is carried out up to second order. Also, the parametric finite element modeling of the problem is done by using ANSYS Parametric Design Language (APDL). The effect of variations of cylinder’s geometric, loading, material and heterogeneity parameters upon nonlinear behaviour of the shell is studied. The results are validated and compared with those obtained from the finite elements method (FEM).