PbS is a direct bandgap IV-VI semiconductor with a bandgap of 0.41 eV and an exciton Bohr radius of 18 nm at room temperature. It has exceptional third-order nonlinear optical properties and applications for optical devices. In this paper, we synthesize PbS nanoparticles in aqueous solution using different capping agents. UV-Vis spectra show that the sample is much more stable. The nanoparticles are investigated by UV-Vis absorption spectrum and X-ray diffraction pattern. As a result of the nanoparticle size reduction, we observe a larger blue shift in absorption spectrum. Using the Scherre equation, the mean particle size is estimated about 8.2 nm.The Z-scan technique, which is a single beam method, is used to investigate the nonlinear optical properties of the as-prepared particles. We use a continues He-Ne laser with 632.8 nm wavelength in the set-up. The method is applied to three samples with different molarities and the nonlinear refractive index is calculated to be of the order of 10-7(cm2/W) with a negative sign.