Welding vessel objects such as heat exchanger of power plant, oil tank and other civil-used pressure vessels are comparatively large size products. Design works for such products concern with the geometric design data, more improtant aspect is to treat with design knowledge in design operations. However, traditional CAD systems generally depend on shape-oriented approaches and pay less attention on the features of design activities and how to deal with design knowledge used in design process. To solve such a problem, as an integrative frame to deal with the knowledge of design object and design process, there has been considerable growth of interest toward the knowledge-based design support systems. We have proposed an object oriented design object model for developing knowledge-based manufacturing design system of welding vessel objects. In this model, assymbly structure of a design object is represented by the combination of the essential design operation elements and their dependencies. This paper discusses the design object modeling in manufacturing design of welding vessels as well as its application in welding estimate of a welding vessel and method of dealing with the design knowledge related.