Theoretical equations for hoop stress, longitudinal or meridian stress and Von Mises stress of an axismmetric conical pressure vessel and a frustum of conical pressure velle, both of which are pressuized by hydrostatic loading, are derived from equilibrium equations. The membrane stresses conputed by theoretical equations for a conical pressure vessel and a frustum of conical pressure vessel are compared with the values obtained from finite elelment method. Based on the fact that the computational values by theoretical equations are well agreed with the finite element results, derived equations are proved to be valid and it is possible for those equations to be conveniently used for structural analysis or design of frustum of conical pressure vessel which is a part of silo body.