The structure of design knowledge appropriate for the design tree representation of design plan is presented in this thesis. Structuring of the knowledge base during the design process is based on the analysis, and then synthesis of the factors important for the realization and improvement of the design process. The performed analysis of the design process and information transformation is based on three approaches to the design process, relevant for the systematic research of knowledge structure. The paper presents the analysis and a brief overview of the situation in the field of applying the artificial intelligence methods in designing, with reference to the techniques of knowledge presentation and implementation. It also gives a presentation of the design process based on a plan, as well as the structure and syntax of plan notation, and an explanation of the aspects of the plan exploitation. Action functions, which are operators of design plan nodes, are treated as the basic parts of the design knowledge, which, when activated, transforms a set of information. The thesis also provides a concept and structure of design knowledge, and suggests a model for developing a base of technical knowledge, based on the system of action functions. For the field of mechanical engineering design, mainly machine parts and their functional assemblies, the knowledge structure is outlined, which should provide the basis for developing a technical knowledge base. Each group of machine parts is separated according to the criterion of the basic technical principle, thus allowing a systematic and quick search of the field of knowledge regarding machine elements using the suggested structure of the unit action functions.