The title compound, LaMn₂O_5+δ was prepared by a mild hydrothermal method at 240 °C for 24 h. XPS and TG analysis show that the mean valence of Mn in LaMn₂O_5+δ increases with the increase of KOH concentration in solution. XRD and EDX results indicate that pure LaMn₂O_5+δ can be obtained only when the molar ratio of starting materials KMnO₄ and MnCl₂ is close to 3∶7. Dc polarization experiments and impedance spectrum analysis indicate that LaMn₂O_5+δ shows mixed oxygen ionic and electronic conductivity at high temperature. The electrical conductivity increases with the increase of mean valence of Mn in the products. The total conductivity for LaMn₂O_5.148 is 1.82 × 10⁻² S cm⁻¹ in air at 600 °C.