The thermal conductivity of ultrananocrystalline diamond/hydrogenated amorphous carbon composite films prepared by coaxial arc plasma deposition (CAPD) was measured by a timedomain thermoreflactance method. The specific heat and density of the films were measured to be 0.891 J/gK and 1.70 g/cm3, respectively, as parameters for the estimation of the thermal conductivity. Based on them, the thermal conductivity was estimated to be 2.32 W/m-K. This value is smaller than that of general UNCD/a-C:H films prepared by chemical vapor deposition (CVD). It might be because the CAPD films possesses a huge number of grain boundaries owing to extremely small grains of approximately 2.3 nm as compared with those of CVD films and its structure enhances phonon scattering.