Infrared detectors are of extensive interest due to minimum atmospheric absorption and are mainly developed in the narrow wavelength region (08–12 μm) III-V group compounds. Indium Antimonide (InSb) is a narrow bandgap semiconductor for a vital purpose. We report here the structural and optical studies of the Fe concentration variation in In1-xFexSb (x = 0.00, 0.05, 0.10 and 0.20) bulk systems. Rietveld refinement of the X-ray diffraction pattern for x = 0.00 reveals the single-phase having space group F4¯3m with Face centred cubic structure. The crystallite size observed from the Debye Scherrer formula decreased with the increase in Fe concentration. The optical studies are examined by FTIR absorption spectroscopy. The energy bandgap evaluated for doped samples is 0.14 eV. The doping of Fe in the InSb bulk system reduces the optical energy bandgap.