aMnO3 ultrafine powders were prepared by pyrolysis of amorphous citrate.XRD indicates the single phase of LaMnO3 has been synthesized. They are hexagonal in the structure. The experiments show that the mean particle size of LaMnO3 increases greatly with the increase of firing temperatures. The change of the electrical resistivities with the particle sizes has been first studied for the LaMnO3 ultrafine powders. It was found that the electrical resistivites of LaMnO3 at room temperature decrease with the increase of particle sizes, the activated energy for conduction decreases with the decrease of the particle sizes.