The combination of Artificial Intelligence (AI) and Computer Aided Design is a trend in computer aided techniques. In this paper, we discuss some AI techniques used in special CAD systems developed at Zhangjiang University. First, an expert system used in art pattern CAD is presented. Accessing the knowledge bases, the expert system reasons to produce various kinds of pseudo-3D art patterns which can be used for textile such as cotton, silk, and carpet. We shall focus on its knowledge representation and fuzzy reasoning mechanism. In the second part of the paper, a geometric modelling system for architectural CAD is described; and in particular, a method to construct the hierarchical geometry model of complex scene environment is introduced. The method is based on the clustering technique of Pattern Recognition. The geometric modelling system can build the hierarchical 3D geometric model automatically and the efficiency of the system is obviously improved with the method.