Abstract: Design unit consists of an hierarchy of "function units" conforming to the functional requirements of a mechanism.Using the method of design unit can simplify the design of complicated kinematic scheme,it will suffice to select an appropriate function unit for each design unit.On the basis of that,this paper establishes a design model for presenting the kinematic scheme of machinery which can be recognized by a computer and promotes to solve the key problem of automated kinematic scheme design.Since the description of function unit is a basic clue to facilitate its matching with the corresponding design unit,an object-oriented approach to describe the function unit together with its subordinate sub-function units by the mode and direction of motion,speed rate,execution elements,sub-function structure and a set of relevant "operation" to generate the description.