To deal with the functional relationship between mechanical product subsystems, a structural similarity computation model of product system by using directed fuzzy graph was proposed for product case retrieval during conceptual design. The concept of directed fuzzy graph was defined, and a directed graph model of product system was created and then represented by permanent matrix. Structural analysis of product system was conducted by using permanent function and the structural similarities of product system directed models were computed by combining subjective and objective weights. Finally, the case retrieval of SF2005 proved the applicability and effectiveness of the proposed similarity computation model.