This research discusses a technique for the synthesis of ultrafine LaBO3 (B: Co, Mn) and La1-xSrxBO3 with a perovskite structure, in which a solution of Na2CO3-NaOH was adopted as the precipitating agent and the supercritical drying method was applied. By means of this technique, the mean particle size obtained was about 15nm and its corresponding specific surface area was 30m2g-1. The activity for CO oxidation over LaBO3 increased significantly with a decrease of its particle size. In addition, the substitution of strontium for lanthanum in nanometers LaBO3 increased the activity.