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The effect of Al2O3 and Cr2O3 additives was investigated on Curie temperature, lattice constants, magnetic properties and particle shapes of the calcinated Sr-ferrite, and on the permanent magnet properties of the sintered Sr-ferrite. With increasing the amount of Al2O3 additives, Curie temperature was decreased, the lattice constant was contradicted and the intrinsic coercivity was considerably increased. In case of Cr2O3 additives, Curie temperature was slightly decreased, but little change was observed on lattice constant. The intrinsic coercivity was slightly increased. In terms of the additive effects, the intrinsic coercivity of the sintered Sr-ferrite showed a similar tendency to that of the calcinated Sr-ferrite. It could be concluded that the Al2O3 additive controlled the intrinsic coecivity by forming solid solution with Sr-ferrite, and Cr2O3 additive behaves to control the grain growth of Sr-ferrite. By simultaneous adding of SiO2 and Al2O3 or Cr2O3 additives in pre-calcinating stage, we have cleared the hurdle of obtaining the high quarity magnets of Br=4.1 KG, iHc=4.0 KOe and or Br=3.8 KG, iHc=5.0 KOe, which were keenly claimed by car motor makers.