Well-crystallized cuprous oxide (Cu2O) nanoparticles were successfully fabricated using a microemulsion (ME) method. A solution containing copper (I) element for hydrolysis reaction in small water droplets was obtained using copper (I) acetate as the raw material with the appropriate addition of chelating agents (n-propylamine or benzylamine). A transparent yellowish suspension containing Cu2O nanoparticles (ME-Cu2O suspension) was prepared at room temperature, using cyclohexane or benzene as a hydrophobic solvent. The average sizes of the Cu2O nanoparticles obtained in this case were estimated to be 3.9 and 6.2 nm, respectively. Cu2O thin film (average size=24.6 nm) and a hybrid composite of Cu2O nanoparticles with a polymer matrix (average size=5.5 nm) were also prepared from a ME-Cu2O suspension. Optical absorption study revealed that direct band gaps of the Cu2O thin film (2.50 eV) and the hybrid composite (2.65 eV) were much greater than that of bulk Cu2O (2.17 eV), as a result of quantum size effect in Cu2O. The photoluminescence of the obtained Cu2O nanoparticles was measured at 12 K. The present study provides a new promising method to fabricate well-crystallized Cu2O nanoparticles of single nanometer sizes.