Although studies on the Fe3O4 magnetic particle formation have been markedly advanced by the strenuous efforts of numerous researchers, the preparation and formation of ultrafine Fe3O4 particles with narrow size distribution still remain as an open area due to their broad applications, especially in biomedical fields such as target drug delivery and medical imaging. A new method for the titled template-mediated synthesis of Fe3O4 ultrafine magnetite particles (several tens of nanometers) with narrow size distribution from a polymer matrix has been investigated. Hydrazine hydrate was used as a reducing agent to prepare the poly(itaconic acid-co-acrylic acid) gel-supported iron metallic nanoclusters, which served as nuclei, and were later transferred into ultrafine iron oxide powders in NaOH base solution. The morphology, particle size, structure and magnetic properties, as well as the formation of the particles with narrow size distribution, were investigated by means of transmission electron microscopy, selected area electron diffraction, laser light scattering, wide-angle X-ray diffraction, Fourier-transform infrared spectroscopy, and electron spectroscopy for chemical analysis, as well as magnetic measurements, respectively. It was revealed that the Fe3O4 ultrafine particles prepared from the Fe3+-poly(itaconic acid-co-acrylic acid) complex gel system kept narrow size distribution, which originated from the oxidation and aggregation growth of the primary particles, and the metallic iron nanoclusters would act as “templates” in the later magnetite particles growth stage. This new attempt of template-mineralization of Fe3O4 ultrafine particles in our preparation was proved to be effective for preparing template-mediated ultrafine magnetite particles with narrow size distribution quickly, which could also be used to prepare other kinds of inorganic particles.