CuIn1−xGaxSe2 (CIGS) thin-film has examined as an absorber layer for the solar cells because of the suitable absorption value, stability, and economic in manufacture. CIGS thin films belong to the I-III-VI2 group of the periodic table with the appropriate direct bandgap (1.5 eV). In this study, the CIGS thin films were annealed at ∼200°C for four different annealing times durations (15, 30, 45 and 60 min) to investigate the effect of annealing time on the crystalline structure and optical properties in CIGS thin films prepared by using the sol-gel dip-coating technique. CIGS thin films annealed at ∼200°C for 60 min have been found to have the best structural and optical properties for this study. As the crystallite size increased with the rise of the annealing time the lattice strain decreased indicating the elimination of the crystallite defects in the CIGS thin-film structure. Hence, the structural changes affected the optical properties slightly and the rise of the optical absorbance (A%) resulted in a decrease of the optical transmittance (T %).