The thermal conductivities, λ, of single‐crystal and polycrystalline UO2were measured from 80° to 420°K. The results indicate no observable difference in λ between single‐crystal and polycrystalline UO2, and both materials have broad peaks in λ at ∼220°K. The results were used with literature values to determine the effect of closed porosity on λ. The thermal conductivity of theoretically dense UO2is described phenomenologically from 80° to 1400°K, where conduction is dominated by the phonon component. The phonon conduction is analyzed by comparison with ThO2. This analysis indicates that the high‐temperature λ is limited by 3‐phonon Umklapp scattering processes. Scattering by the disordered spins associated with the paramagnetic U ions contributes a large temperature‐independent phonon scattering term. This mechanism has a mean free path of about 51 Å, which implies that grain boundaries and impurities have a relatively insignificant effect on the phonon conduction far above the antiferromagnetic‐para‐magnetic transition at ∼ 30°K. This implication agrees