The shipbuilding industry is one of the domains which need an effective computer application. Particularly the productivity of process planning of a shipbuilding for crude -oil tanker can be greatly enhanced by introducing CAPP(Computer Aied Process Planning). In this paper we describe a prototype expert system which enables block division process planning in shipbuilding. The system determines block division lines of the midship sections of oiltanker. Case-based reasoning(CBR) approach is applied for this purpose instead of rule-based one.