The perovskite-type oxide La0.6Sr0.4CoO3(LSCO) is noted as a material for electrodes or catalysts. In order to form LSCO powder into a green product using colloid processing such as slip casting, tape casting, extrusion moulding, etc., aqueous LSCO suspensions were prepared and their flow properties were investigated in comparison with those of aqueous YSZ (yttria-stabilized-zirconia) suspensions in this study. The aqueous LSCO suspensions were prepared with LSCO powder synthesized from acetates, ammonium polyacrylate as the deflocculant, and distilled water. The viscosity of the LSCO suspensions system was so similar to that of YSZ system, i.e., a minimum value was obtained in the relationship between viscosity and deflocculant concentration. The Bingham yield stress of the LSCO suspensions increased with aging time. It was found that the amount of Sr ions leached from LSCO powder increased with aging time, too. Consequently, it was revealed that the flow properties are remarkably influenced by the concentration of leached Sr ions. In addition, it was suggested that washing process of LSCO powder is effective in depressing the change of flow properties with aging time of aqueous LSCO suspension.