It is important to understand the behavior of fission products (FPs) for the evaluation of fuel performance. For example, in high-burnup oxide fuels, some FPs dissolve in the fuel matrix and others form oxide or metallic inclusions, which would affect the physical and chemical properties of the fuels. Here, we investigated the thermal conductivity (λ) of oxide inclusions; in particular, we focused on Cs-Mo-O and (Sr or Ba)-Mo-O ternary systems. The λ value of Cs₂MoO₄ is quite low (around 0.6 Wm⁻¹ K⁻¹ at 300 K) compared with that of UO₂ (around 8.5 Wm⁻¹ K⁻¹ at 300 K). In addition, we found that the λ value of (Sr or Ba)MoO₃ is approximately 10 times higher than that of (Sr or Ba)MoO₄. This high λ value of (Sr or Ba)MoO₃ is due to not only a high electronic contribution but also an intrinsically high lattice thermal conductivity (λ_lat). This high λ_lat could be explained using the general lattice thermal conductivity theory; that is, a strong interatomic bonding within a simple crystal structure is realized in (Sr or Ba)MoO₃, leading to an exceptionally high λ_lat compared with that of (Sr or Ba)MoO₄.