Abstract: Under precondition of high emissivity, the thickness of high emissivity ceramic coatings should be reduced to satisfy the requirement for mechanical property, which can also save raw materials and decrease the weight of the apparatus. So the relationship between emissivity and thickness of the coatings is important. In this paper, the coatings were prepared from Al₂O₃ sols and high emissivity fillers by spin and spray processes, respectively. The relationship between emissivity and thickness of coatings was examined, and the critical thickness of coatings was determined while the emissivity changes with the thickness rapidly. Based on Maxwell-Garnett theory, a new theoretical model about the relationship between emissivity and thickness of composite coatings is established. The emissivity values from the theoretical calculation are agree with the experimental data, which shows that the model can be used to predict the emissivity or the critical thickness of the composite coatings.