In the appearance design of a product, the designer should combine his/her creative thinking with the industrial design approaches and skills to carry out the creative design from the aspects of product’s shape, color and texture, etc. However, from the viewpoint of the brand, a company or a product should have its own unique style of appearance. How to balance the creativity of designer and the original style is often a contradiction in product design. Based on this consideration, this paper presented a concept of Inheriting Degree (ID) in product appearance design. The visual characteristics of the product appearance were analyzed first and the hierarchical system for ID in product appearance design was established. The idiographic indexes of ID were given for a product’s appearance in shapes, colors and textures, respectively. By introducing set theory, the mathematical models were established and calculation formulae for each kind of ID were proposed. The results of this paper would provide a theoretical foundation for designer to generate design concepts in brand products. It also would provide a guideline for the development of computer aided design and evaluation system for the appearance design of brand products.