The optimum amount of Sr in La1-xSrxMnO3 for solid oxide fuel cell (SOFC) cathode material was studied by observing the charge transfer resistance, electrical conductivity, and reactivity with the electrolyte. Three transition metals (Cr, Mn and Co) were substituted to the B site of La0.5Sr0.5BO3 and the electrochemical characteristics were investigated by AC impedance method. The electrical conductivity and interface resistance decreased as the amount of Sr increased and the lowest value was shown when the amount of Sr was 50 mol%. The reactivity between the electrolyte and La1-xSrxBO3 (B = Cr, Mn, Co) was investigated and it was found that the secondary phase, La2Zr2O7, was not formed when the substitution amount of Sr was 50 mol%. The activation energy of the electrode was 1.186, 1.761, and 2.043 eV for La0.5Sr0.5BO3 (B = Cr, Mn, and Co, respectively). The calculated capacitance value of La0.5Sr0.5MnO3 was 3.23 x 10(3) muF, so the three-phase boundary (TPB) of this electrode was broader than the other electrodes. Consequently the La0.5Sr0.5MnO3 electrode is appropriate as a cathode material for SOFC due to its broad electrochemical reaction site and its electrochemical characteristics. (C) 2002 Elsevier Science B.V. All rights reserved.