Hollow and filled Cu₂O nanocubes of about 28 ± 5 nm in edge length with a band gap ~2.42 eV have been prepared from cupric nitrate in alkaline aqueous solutions containing fructose and ascorbic acid at room temperature. To the best of our knowledge, this simple strategy demonstrates the first example of preparing high-quality Cu₂O nanocubes (yield>95%) with sizes smaller than 30 nm. By controlling several important experimental parameters such as pH, concentrations of fructose, and molar ratios of fructose/copper (II), different Cu₂O nanostructures were prepared. The cubic nanostructures were evidenced by the metal shadowing and transmission electron microscope (TEM) images. We confirmed that the Cu₂O nanocubes were formed from hollow to filled structures by conducting time-evolution TEM measurements. The thus-prepared Cu₂O nanocubes possess size-dependence absorption and luminescence characteristics; they absorb light at wavelengths 360 and 454 nm and fluoresce at 493 nm (quantum yield 6.6 × 10⁻²%) when excited at 360 nm. A film of Cu₂O nanocubes provided a photocurrent density of ~80 mA m⁻² at a biased voltage 3 V under sunlight illumination (100 mW cm⁻²).