The present paper deals with structural optimal design of cylindrical shells under combined loading against creep rupture and creep buckling. Minimal weight of the shell is the design objective. Under the assumption of constant loading along the axis of the shell, the design objective reduces to the minimal cross-sectional area. Shape of the middle surface of the shell y(x) and the wall thickness h(x, y) are the functional variables. State equations are assumed in the form of nonlinear creep law for an incompressible body. The problem leads to variational optimization with respect to functional design variables under isoperimetric constraints for loading. The set of parameters satisfying a minimum of the cost function is then found numerically.