Disclosed is a method for directly preparing cerium oxide powder in a solution phase by a) mixing a cerium precursor solution with a precipitant solution to cause a reaction; and b) performing oxidation treatment of the reacted solution, wherein at least one kind of pure organic solvent containing no water is used as a solvent for the cerium precursor solution as well as the precipitant solution to thereby prepare the cerium oxide powder, the particle size of which is adjusted to 50 nm to 3 μm. Cerium oxide powder obtained from the method and CMP slurry comprising the cerium oxide powder as a polishing agent are also disclosed. The method makes it possible to prepare cerium oxide powder with an average particle size of 50 nm or greater and high crystallinity, which is difficult to prepare by the conventional wet precipitation process, by using an organic solvent as a solvent in a wet precipitation process, and the so-prepared cerium oxide powder can be used as a polishing agent for CMP slurry even without being subjected to separate heat treatment.