A new methodology was proposed for testing the fracture toughness of YAG transparent ceramics depended on temperature. In our model, the fracture toughness, as a material-specific property, can be expressed as tensile strength treated as the physical property or a material constant. Using this method, a suitable size larger than the inherent defects of ceramic samples and the creation of atomically sharp pre-cracks on the surface of ceramic specimens were able to be ignored. Besides, the fracture of ceramic can be described as the equivalence between the release of elastic storage energy and surface energy associated with the new fracture surface. From thermodynamics theory, the system’s internal energy includes the kinetic energy of atomic motion and the potential energy between atoms in the system. Finally, the fracture toughness at different temperatures can also be calculated by this simple quantitative relationship. In order to confirm the validity of this model, the decreasing fracture toughness of YAG transparent ceramic with increasing temperature was predicted, and the result coincided with the experimental results.