In order to make more progress in Product Definition System for Shipbuilding, curved surface is discussed in this paper. The aims of this paper are 1) to introduce B-spline surface to define ship hull surface and 2) to make clear the concept of plate thickness in our CIM system. The concepts of geometrical model (curves, surfaces) and product model (part, connection, assembly block) are disscussed to implement the system. Especially, data structure due to NIAM (Nijssen Information Analysis Methodology) diagram is used in product model. The system is composed from 1) outer shell surface generator 2) structural design generator 3) Part generator 4) Assembly simulator and 5) Element design. Some examples are shown in this paper.