Abstract: This paper puts the emphasis on studying the optimum design of the thick-walled circular cylindrical pipes combined with GFRP/steel (shortly called, combined pipe, below) subjected to the internal high pressure. Firstly, considering a few of boundary conditions, the formulas of three principal stresses in the wall of the combined pipe in the state for plane strain are derived by elastic theories. Next, the mathematical model for the optimization of the combined pipe is set up, in which the total wall thickness of the combined pipe, tile wall thickness of GFRP out-sleeve and the wound angle of the glass fibre are chosen as design variables, the materials cost of the combined pipe per unit length is taken as an objective function, and the strength requirement of steel pipe and the deformation limitation of GFRP are considered as constraints as well. Finally, the model for the optimization is solved by using the Complex-Methol. Also, the author discusses the influence, caused by the changes of the inner diameter of the combined pipe, the internal pressure as well as the strength class of steel pipe, on the optimum values of design variables. A numerical example shows that, under the same pressure, the materials cost of the combined pipe is less than that of a pure steel pipe, and the weight evidently lighter.