Ultrafine copper particles with diameters in the range of nanometer scale have been prepared in a polymer matrix, using ionic aggregates in Cu2+-random poly(itaconic acid-coacrylic acid) complex precursor films as "microreactors" of controllable size. The morphology and formation of metallic copper particles were studied by transmission electron microscopy and X-ray diffraction techniques. Results from the measurements indicate the copper ions were transformed into nanocrystalline by in situ chemical reduction. It was also found that the polymer chains prohibited the excessive aggregation of the metal atoms and have protective effect on the ultrafine copper particles.