The effect of the temperature and borate structure on the thermal conductivity of the molten CaO-B2O3 binary system was investigated. The thermal conductivity was measured from 1573K to approximately the liquidus temperature using a transient hot-wire method, and was found to decrease with increasing temperature. The temperature dependence of the thermal conductivity in the molten oxide system was interpreted in terms of the lifetime of phonon-like excitations and Maxwell relaxation times. The short-range order borate structure was investigated through magic angle spinning (MAS) and triple-quantum MAS (3Q-MAS) NMR. Although the content of 4-coordinated boron was not significantly affected by the addition of CaO, structural changes associated with the fraction of 3-coordinated boron in ring and non-ring sites were observed. The Raman spectra showed that the intermediate range order borate structure was changed from boroxyl rings to chain-type metaborate units at higher CaO concentrations. The thermal conductivity decreased with decreasing BO1.5/CaO ratio, owing to the depolymerization of the borate network.