Indium sulfide thin films consisting of In2S3 and In6S7 phases were synthesized onto microscope glass substrates with different nanoparticle size using the chemical bath deposition method (CBD). Immediately after obtaining the films, they were annealed at 400 degrees C for 1 h in reduced media in order to get better crystallization. The effect of nanoparticle sizes on the structural, compositional, optical, and electrical properties of the films was investigated. The films were characterized by x-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive x-ray spectrometry (EDS), UV-Vis spectroscopy and sheet resistivity measurements. The XRD spectra revealed the existence of both the cubic In2S3 and monoclinic In6S7 phases. From the SEM micrographs, the deposited films showed dense and good coverage of the surface with cracks. Moreover, nanoparticle sizes increased from 53 nm to 142 nm with increasing deposition time as well as film thickness. With an increase in nanoparticle size, the S/In ratio in the films decreased from 1.74 to 1.21 showing sulfur deficiency in the deposited films. The direct band gap (E-g) of the films decreased from 3.35 eV to 2.70 eV with increasing nanoparticle size. The sheet resistivity of the films decreased from 1.69 x 10(7)Omega/Sq to 4.61 x 10(3)Omega/Sq. The obtained results demonstrate that nanoparticle sizes effected the structural, compositional, optical and sheet resistivity of the films. (C) 2020 Elsevier B.V. All rights reserved.