By use of function-behavior-structure-shape model, an analogy design method for mechanical products based on structure similarity is presented. Structure model includes qualitative models of symbol part structure and product assembly structure. It is an information carrier to connect conceptual design and detail design. The main aim to set up structure model is to realize design reuse. By use of the structure similarity between source design and target design the product design is picked-up. The 16 types of design processes based on analogy are present. Finally structure design modeling is divided into three levels, namely symbol modeling, structure modeling and feature modeling. The structure hierarchical modeling method is discussed.