The present invention relates to a method for preparing magnetic powder comprising homogeneous and fine particles using an alkali-producing enzyme. The object of the present invention is to provide a method suitable for preparing magnetic powder comprising relatively small particles, for instance, fine particles having a particle size ranging from 50 to 500 nm. The present invention relates to a method for preparing at least one member selected from the group consisting of iron oxides, iron hydroxides and iron oxyhydroxides which comprises the step of alkalizing a solution containing iron ions utilizing an alkali-producing enzyme and a substrate of the enzyme.According to the present invention, there can be produced magnetite (Fe.sub.3 O.sub.4) and maghemite (gamma-Fe.sub.2 O.sub.3) useful as magnetic powder as well as goethite (alpha-FeO(OH)), hematite (alpha-Fe.sub.2 O.sub.3) and lepidocrocite (gamma-FeO(OH)) useful as the starting materials thereof. The magnetic powder can be used as the materials for magnetic recording and magnetic fluid; carriers for bioreactors; those for magnetic separation of cells and biopolymers; and those for microcarriers of medicines.