The variation of the stable region of lead ion in aqueous environment has been studied according to the temperature change based on its stable region at standard state by constructing Pourbaix diagrams at different temperatures. The temperature range considered was between 278∼338 K and the stable region of Pb^2+_(aq) was observed to decrease as temperature rises, which was due to the decrease of the stable region of water and the increase of the stable region of solid lead hydroxide. The solid lead compounds considered were PbO_(s), Pb_3O_4(s), and PbO_2(s), and the pH at the boundary between the stable regions of PbO_(s) and Pb^2+_(aq) was observed to be lowered with temperature. For Pb_3O_4(s) and PbO_2(s), their stable regions were estimated to be enlarged as temperature is raised. Considering the change of the stable region of lead ion with temperature, in the application of the precipitating method for the treatment of lead ion in water it was considered to be more desirable to treat lead ion as its hydroxide form rather than its elemental form at higher temperature.