The protective ability of rust layers formed on weathering steels exposed in an industrial region for various periods has been studied by the potential measurement, and the rust composition and structure were also studied. Potential of weathering steel in 0.1mol/L Na₂SO₄ solution tends to become noble with exposure time. Potential of weathering steels with rust layers, which were exposed for a long time and had superior atmospheric corrosion resistance, became over -0.3V (vs. SCE). The rust layer possesses the double layer structure composed of inner dark layer of α-FeOOH and outer bright layer under reflected polarized light, which is observed typically in the protective rust layer. On the other hand, potential of mild steel exposed for a long time is less noble at about -0.45V (vs. SCE). These results mean that potential reflects the difference of characteristic of rust layer. Potential became noble with increasing amount of α-FeOOH in rust, which suggests that potential depends on the composition of the rust layer. Furthermore, potential correlates with the ratio of rust composition α/γ*: α-FeOOH/(γ-FeOOH+β-FeOOH+Fe₃O₄), and potential of -0.3V (vs. SCE) corresponds to α/γ*=2. Therefore potential as well as the ratio α/γ* is able to use for the evaluation of the protective ability of the rust layers formed on weathering steels.