A thermal property parameter for expressing the ground heat flux (cρλ; product of the heat capacity and the thermal conductivity) of urban complex terrain was estimated. The surface temperature time series was observed during nocturnal radiative cooling, and employed in the estimation. The surface temperature was obtained using the airborne-measured upward longwave radiation in order to consider the directional anisotropy of radiometric surface temperature. The effective thermal property parameter for a town-scale urban area was found to be two to four times larger than that of the surface material component. The explanation for this, shown by several model simulations and other radiometric observations, was total surface area increased due to urban canyon structure. The parameter cρλ on the town-scale was expressed by the canyon shape, and the cρλ of its component material.