Ultra-small PbSe quantum dots (QDs) were synthesized using conventional hot-injection method. A small amount of Sn was used as a nucleation promotion agent to control nucleation and growth during the QDs synthesis process. The average diameter of the QDs is about 1.6 nm, of which absorption peak centers at 550 nm and photoluminescence peak centers at 750 nm under 350 nm laser excitation with power as low as 500 μW. Transmission electron microscopy images confirm that the QDs size well matches with the calculated diameter from Brus equation. This match and electron energy loss spectroscopy analysis proves that Sn is not involved into the final structure of the ultra-small PbSe QDs. An ion-exchange process was proposed for the nucleation control and ultra-small QDs synthesis. The prepared ultra-small QDs could be a promising candidate for luminescence, solar cell devices, and others.