The secondary structures of Cu2O nanoparticles were prepared in aqueous solution utilizing self-assembled aggregation process. By introducing polyacrylamide (PAM) as a secondary surfactant, the colloidal nanoparticle aggregates (CNAs) become uniform in size and exhibit spherical shape compared to the random aggregates without PAM. The size of CNA can be systematically controlled from 300 nm to 1000 nm by varying PAM concentration. The formation mechanism was explained based on a conventional colloidal particle formation mechanism. These control methods may generally be applied to the preparation of secondary nanoparticle structures. (C) 2009 Elsevier Inc. All rights reserved.