The major mechanism in noninvasive surgery using HIFU is thermal ablation. Extensive research was conducted in order to obtain temperature mapping of the procedure, however, this is not the only important feature in monitoring thermal ablation. The thermal diffusivity at the ablation site also plays an important role in the final therapeutic outcome, as it influences the temperature's distribution achieved in the tissue in space and time. Moreover, tissue thermal diffusivity is different in tumors as compared to normal tissue. This study presents a method for estimating, non-invasively, the thermal diffusivity using Through Transmission (TT) ultrasound imaging and HIFU heating. A computed ultrasound system for scanning the tissue in a TT mode was used during its cooling process. The experiments were conducted on a tissue phantom made of Agar and on porcine fat.  A HIFU system was used for heating the tissue. The temperature was kept lower than  to avoid irreversible damage. TT scanning provided the time of flight (TOF) values of a chosen area in the tissue. The TOF values were consequently converted into average values for the Speed of Sound (SOS).  The average SOS profiles, during cooling, were registered. A model for evaluating the spatial distribution of the SOS and its relation to temperature, in one slice of the tissue, was developed. Using the average SOS profiles along with the model, the changes in temperature profiles over time were estimated. These changes in the temperature profiles were then used for calculating the corresponding thermal diffusivity of the studied specimen. The results have demonstrated the ability to assess the thermal diffusivity non-invasively using TT ultrasound. The empirically obtained values matched the reference values found in the literature and values obtained by a simple heat transfer experiment. Thermal diffusivity for porcine fat was found to be one order of magnitude lower than the one for agar. This stems from the fact that fat is a natural insulating material. The fact that the results for agar and porcine fat were significantly different indicates that thermal diffusivity may also be used for characterizing tissues and for differentiating between different tissue types. This method may potentially be used as a diagnostic tool for breast tumors characterization and for success assessment of thermal ablation treatments.