This paper deals with "Computer Aided Chair Design System" based on form features of chair elements that constructs the 3-D product model of a chair from a sketch drawn by a designer. In design processes of a chair, a designer usually wants to express his/her idea of the form and structure of a chair by drawing sketches because sketching is one of the most desired manners for expressing designer?s idea. Besides, CAD data of a chair are essential in its manufacturing processes. In this study, a system is proposed to construct the 3-D CAD data of a chair by arranging form features of chair elements with fitting them on a sketch. The system enables designers to easily make the 3-D chair models from their idea sketches and to evaluate the design-form by figures from the various visual angles.