We report the electrical and thermal properties of β-pyrochlore (defect pyrochlore) oxides AFe W O (A = K, Rb, Cs) with a crystal structure having a small cation surrounded by oversized cage-like framework. The thermal conductivity, κ, of CsFe W O and RbFe W O showed extremely low values as oxides (below 1.0 W/mK) similar to those of ATaWO (A = K, Rb, Cs) which we have already reported. These low κ values are ascribed to a “rattling” motion of the A cations, evidenced by their crystal structure refinement and the Raman spectra. Their electrical conductivity, σ, was in the order of 10  S/cm, and the Seebeck coefficient, S, was −500 to −600 μV/K. The electrical conductivity of AFe W O (A = Rb, Cs) was much higher than those of ATaWO (A = Rb, Cs), suggesting that an appropriate selection of the framework composition enables us to have better thermoelectric performance. 0.33 1.67 6 0.33 1.67 6 0.33 1.67 6 6 0.33 1.67 6 6 −3