Abstract: A sketch-based 3D CAD system is presented in the paper, and a group of sketch gesture rules and sketch semantics automatic machines construct the architecture of the system. It describes the method of capturing, expressing and understanding the sketch intent. This method makes the system more effective during 3D CAD conceptual design. The prototype system uses a grip pen and a digital tablet as its input devices, and it can construct some complicated 3D solid body and 3D scene in a natural and rapid way.