The effect of hollow structure on the optoelectronic properties of β-Fe 2 O 3 hollow nanoparticles (HNPs) was determined. Spectrophotometry showed that the optical transmittance of the β-Fe 2 O 3 HNPs was less than 40% in the visible-light region. This opaqueness was suggested to be an optical characteristic, commonly found in the authors' previous studies of TiO 2 and δ-Al 2 O 3 HNPs. In addition, β-Fe 2 O 3 HNPs had a band gap (1.86 eV) between amorphous (1.73 eV) and polycrystalline (1.97 eV) β-Fe 2 O 3 thin films, which was a 5–7 nm thick shell that embraced an intermediate volume of the crystal phase, in-between the two thin films.