Cu-In-S nanocrystals (CIS NCs) were prepared by thermolysis of a mixed solution of copper(II) acetylacetonate [Cu(acac)₂], indium stearate [In(St)₃], and dodecanethiol (DDT) in noncoordinating solvent 1-octadecene (ODE) at 250 ℃ for 50 min. The diameter of rod-like CuInS₂ nanocrystals was about 2 nm, and the length of 70 nm. The microstructure of CIS NCs was determined to be of the tetragonal phase. The microstructure, morphology, and phase constituents of the CIS NCs were examined using diverse techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The third-order optical nonlinearities of rod-like CIS NCs in n-hexane were studied by Z-scan technique at a wavelength of 770 nm. The results showed that the third-order optical nonlinear refractive index γ, absorption coefficient β, and susceptibility χ⁽³⁾ were determined to be 1.76×10^－17 m²/W, 7.95×10^－11 m/W, and 8.60×10^－12 esu, respectively.