LaFeO3 with 0.1 Mg substitution at the A-site (La0.9Mg0.1FeO3) and B-site (LaFe0.9Mg0.1O3) has been successfully synthesized via sol-gel method. The crystal structure, morphology, and optical properties of La0.9Mg0.1FeO3 and LaFe0.9Mg0.1O3 powders were characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Ultraviolet-Visible (UV-Vis) Spectroscopy, respectively. Based on the Inorganic Crystal Structure Database (ICSD) No. 98-002-8255, the XRD patterns of both samples possess an orthorhombic structure with Pn m a space group. Optically, substituting Mg at both A and B-sites influences absorbance and reflectance in the wavelength range of 350-800 nm. Moreover, by the Tauc Plot, we determined the optical band gap energy of La0.9Mg0.1FeO3 and LaFe0.9Mg0.1O3, which are 2.43 eV and 2.30 eV, respectively. © Published under licence by IOP Publishing Ltd.