This paper presents an object-oriented model of design process and an object-oriented design support system which assists the basic design of machine tools. The process of the basic design of machine tools is analyzed in the case of machining centers. The design process is represented by a series of phases, each of which consists of two steps. The first step is to select the design parameters and the second step is to determine the values of the design parameters. The "design object" is defined here as encapsulated information which is dealt with through phases of the design process. The model of the design process is represented by trees and their relations. A tree represents the generalization-specification relation of design objects which are nodes of the tree. The relations among the trees constitute the whole-part relation. A design support system is developed based on the object-oriented model of the design process.