The present invention relates to a method for preparing single crystalline cerium oxide nanopowder capable of easily controlling the shape and granularity of particles. The method includes preparing cerium hydroxide by precipitating a cerium salt in the presence of a solvent mixture of organic solvent and water and hydrothermally reacting the prepared cerium hydroxide. The prepared single crystalline cerium oxide has a particle size of not less than about 30 nm and an excellent degree of dispersion.