The purpose of this paper is to study the methodology on structural optimization of inertially stabilized platform (ISP). It is difficult for the ISP structure design since lots of objects are required and many of them are conflicted each other, hence structural optimization should be carried out to seek a compromise between theses contradictions, such as high stiffness and light weight, high ratio of payload to mass and small size, etc. In this paper, first of all, an optimal configuration of 3-axis gimbals assembly is determined on the basis of analysis on the operating environment; and then, an approving structure is eventually obtained with the help of 3D CAD modeling and FEA. The structure is compact as well as lightweight. FEA results show that its strength and stiffness are both satisfied with the requirements.