Glossy appearance is commonly observed on natural and fabricated objects. This paper proposes a physically-based model to account for this behavior. Our basic consideration is that a glossy surface typically consists of a thin transparent layer above the substrate material. We assume that the top surface is very smooth and the interface between the transparent and substrate materials is a Gaussian height field. The numerical results and rendered images generated using our model have successfully demonstrated the features of glossy appearance. This work has eliminated the arbitrariness on choosing parameters in previous empirical method such as Phong model. Since all involved parameters are physically measurable, our model can be strictly verified by experiment. This model has a capability of achieving high accuracy for applications in computer graphics, computer vision, color science, and appearance design.