Au2S nanoparticles (NPs) were prepared in the cavity of the cage-shaped protein, apoferritin. Apoferritin has a cavity, 7 urn in diameter, and the diameter of fabricated Au2S NPs is about the same size with the cavity and size dispersion was small. Although Au2S is generally insoluble in water, our prepared Au2S NPs can be dissolved over 100mg/mL in water due to the water-soluble protein shell. This solubility will make it possible to place and construct nanostructures made of Au2S NPs.