. Deployable truss has a wide range application in aerospace industry. Two important issues in the design of all deployable space structures are the weight and its stiffness in the deployed state. This is because lower weight of deployable truss can greatly save the cost of launching and higher stiffness can improve the loading capacity. This paper aims to design a tapered deployable mast with lower weight and higher stiffness. The mobility analysis and geometry condition is first introduced. Then structural optimization problem with mass as its objective is solved by using ANSYS Parametric Design Language (APDL). The optimal results show the proposed tapered mast has excellent performance on mechanical properties by comparing with normal mast with identical cross section. Finally, some possible applications and problems are described.