This paper presents a new mathematical model for determining thermal diffusivities of very common solid geometrical objects such as an infinite slab, infinite cylinder, or sphere cooled in a medium. In practice the model will be implemented by using the center temperature measurements of these solid objects, and then the cooling coefficients will be obtained. The cooling coefficient, which is the most important process parameter, has a strong effect on the thermal diffusivity. An illustrative example of the use of the model is presented. In addition, new characteristic equations of the transient heat transfers for these geometrical objects were developed. The results of this study show that the present model can be considered as a simple and efficient tool for determining thermal diffusitivies of geometrical objects subjected to cooling.