In this MS thesis a model of user interface for plan based design of mechanical assemblies is proposed. The research directions of the user interface development are investigated, especially considering design process requirements. State of the art, models and approaches in the development of the graphic user interfaces are considered. The methods of the plan representation of design process are discussed. The Model of “intelligent” CAE system is also described. Role of the user interface in such CAE system is explained and the main parts of the user interface are determined. The structure and the parts of the model of the user interface are presented and programming methods used in the development of the model are described. The model of designer oriented user interface is proposed and a prototype of the model is developed. The prototype of the user interface model interacts directly with scenario database.