Ultra-fine particles of La0.5RE0.1Sr0.4MnO3 (RE: Y, Dy, Sm or Ce) with perovskite structure were prepared by the co-precipitation method, resulting in high surface area and good thermal stability catalysts (S-BET reached 45 and 16.5 m(2)/g, upon calcination at 700 and 1000degreesC, respectively). The thermal stability of these ultrafine particles was effectively improved with partial substitution of RE3+ (Y3+, Dy3+, or Sm3+) for La3+ in La0.6Sr0.4MnO3. The catalysts exhibited high activity for the total combustion of methane. For the catalysts calcined at 1000degreesC, La0.5RE0.1Sr0.4MnO3 (RE: Y or Dy) were much more active and showed much higher specific surface area than La0.6Sr0.4MnO3.