Abstract: The cathode LSM powders of solid state fuel cells (SOFC) were manufactured from solid state reaction materials such as La₂O₃, MnO₂, and SrCO₃. The modern instrument, such as XRD, TGA/DTA etc, were used to research the processing steps of LSM fine powders. During the processing, there was La₂SrO_xexisted, no LaMnO₃. The reaction steps were described as follows. First, La₂O₃+SrCO₃→La₂SrO_x+CO₂, andthen La₂SrO_x+MnO₂→La_1-xSr_xMnO₃, finally got La_1-xSr_xMnO₃. This result is different from the recognizedsteps during the LSM processing, in which La₂O₃+MnO₂→LaMnO₃ first, and then LaMnO₃+SrCO₃-La_1-xSr_xMnO₃.