Cuprous oxide (Cu 2 O) nanoparticles and microparticles have been prepared by liquid phase chemical synthesis. The samples were characterized by means of SEM, XRD, UV/DRS and XPS. It was presented that as-prepared Cu 2 O nanoparticles are substantially stable in ambient atmosphere and the Cu + as main state exists on the surface of Cu 2 O nanoparticles. As-prepared Cu 2 O microparticles can exist stably as a Cu 2 O/CuO core/shell structure; and the Cu 2+ as main state exists on the surface of Cu 2 O microparticles. The behaviors of adsorption, photocatalysis and photocorrosion of Cu 2 O particles with different sizes were investigated in detail. The results show that Cu 2 O nanoparticles are very easy to photocorrosion during the photocatalytic reaction, which cannot be used as photocatalyst directly to degrade organic compound, although as-prepared Cu 2 O nanoparticles exhibit special property of adsorption. Cu 2 O microparticles have a higher photocatalytic activity than Cu 2 O nanoparticles because of its slower photocorrosion rate, although Cu 2 O microparticles have much lower adsorption capacity than Cu 2 O nanoparticles. The mechanisms of photocatalysis and photocorrosion for Cu 2 O under visible light were also discussed.