The conceptual design model for mechanical product and its computer aided design are the important and focus problems. A mechanical product design process of FPBS model, function-principle-behavior-structure model, was put forward and discussed. The semantic relationships between the elements of FPBS model were studied to make foundation for the application of FPBS model. The operational demand of knowledge system for FPBS model was analyzed, and indicates the ontology can be applied effectively to knowledge system operation of FPBS model. Then, an ontology based FPBS model was established and a FPBS ontology model system was constructed. The experimental results indicate the model can be used in computer aided conceptual design for mechanical product effectively.