Thermal diffusivity, heat capacity, and thermal conductivity of Al 2 O 3 Ni composite with 5 vol.% of Ni particulate were investigated. The thermal conductivity of the composite increases monotonically with increase of Ni particle size, and approaches a limiting value for Ni particles of 2 μm or larger. The reduced conductivity at smaller particle sizes is due primarily to the increased interfacial resistance related to the increase of Al 2 O 3 /Ni contact area, even though metal Ni has higher value of thermal conductivity than that of Al 2 O 3 . The mode of heat conduction in the composite with the smallest Ni particulate, i.e. D = 0.1 μm, is a result of both phonons and interfacial defects, and this leads of the composite characterizing with a lower temperature-dependent thermal conductivity. The Kapitza radius of the composite was experimentally determined as approximately 0.6-0.7 μm, at which the Ni conductivity is balanced by interfacial resistance, resulting in composite conductivity = matrix conductivity. This suggests that a minimum Ni particle size of ∼ 1.4 μm is essentially required for the improvement of the overall thermal conductivity in Al 2 O 3 Ni composite.