The importance of MBSE and MBD together with the difference between modelling in systems engineering and modelling in design is discussed. Design has to be carried out on a deterministic and unambiguous base, even it may go through a zig-zag process. In comparison with the modelling in systems engineering which tolerates the use of some natural language, the modelling in design has to adopt modelling language rigidly to express its results. As the design is a process of knowledge flow, knowledge integration, knowledge competitiveness and knowledge evolution, and as it must simultaneously meet the requirement of function on material, the requirement of function on mental and the requirement of function on social, a set of suggestions on classifications and expressions of function knowledge in design are presented as follows. 1) transfer functions are used to express the knowledge of magnitude of function elements; 2) block diagrams formed by blocks, input vectors, output vectors, controlling vectors and operators are used to express the relationship between function elements; 3) transfer functions are derived from inputs and outputs of function elements. When the design knowledge is supplied in a distributed source environment by subdivided and/or specialized design knowledge services the deterministic and unambiguous expressions of behavior knowledge and structure knowledge are sometimes necessary. It is, therefore, meaningful to combine the previous suggestions concerning expressions of function knowledge with any part of any systems engineering modelling language which can determinately and unambiguously describe the behavior and structure of a function element or a system in design.