Photoabsorption properties of beta-FeSi2 nanoislands epitaxially grown on Si(111) and Si(001) have been discussed using photoabsorption nano-spectroscopy based on scanning tunneling microscope. The obtained spectra exhibit clear features around 0.86-0.91 eV and around 0.71-0.74 eV, which are explained as a direct and an indirect photoabsorption edge of beta-FeSi2, respectively. We also observed a blue shift of spectrum obtained from beta-FeSi2 nanoislands on Si(111) substrates, compared to those on Si(001) substrates. We attributed the dependence on Si-substrate orientation not to a quantum confinement effect but to an effect of elastic strain in the beta-FeSi2 nanoislands epitaxially grown on the substrate. (C) 2011 Elsevier B. V. All rights reserved.