A facile and room temperature route for the production of very small, nearly monodispersed nanocrystalline lead sulfide (PbS) by ultrasonic irradiation in aqueous solution are presented. In this process, 3-mercaptopropionic acid (3-MPA) was used as a capping agent. The X-ray diffraction (XRD) studies display that the products are well crystallized in the form of cubic structure. The morphology of as-prepared nanoparticles was characterized by scanning electron microscopy (SEM).The optical absorption studies also show the band gap of PbS material was blue shifted due to quantum confinement of charge carriers in quantum sized volume. A possible formation mechanism for the production of nanostructures in the aqueous solution of the product with the aid of ultrasonic irradiation is proposed.