A CAD approach which can optimize and automate the parting direction determination is presnted. The approach is based on the geometrical and topological information of the solid modelling of the plastic moulded part in order to select a pair of optimal parting directions of a two-plate mould which minimizes the number of side cores. The shell of a part is divided into inter-influential regions and non-influential faces in the mould design point of view. Through analyzing and computing the accessibility direction cones of the inter-influential regions, the optimal parting directions can be determined automatically.