The relationship between microstructural damage and the thermal diffusivity of unidirectional reinforced Nicalon™-LAS II was investigated. Damage in the form of matrix-microcracking and fiber-matrix debonding was induced in the composites through Monotonic and cyclic mechanical loading. The thermal diffusivity of the composites was measured in directions transverse and parallel to the fiber axis by the standard flash diffusivity method. The results showed that damage induced by mechanical loading only affected the thermal diffusivity parallel to the fiber direction. Mechanical loading followed by oxidation of the carbonaceous interface resulted in significant changes in the longitudinal and transverse thermal diffusivity of the mechanically-damaged samples. These experiments showed the important role of the interfacial conductance on both the longitudinal and transverse thermal Diffusivity. The experimental data, along with finite element calculations, were used to assess the use of micromechanics-based models in predicting an effective thermal conductivity of damaged composites.