Five colors water-soluble CdTe QDs are synthesized with thioglycolic acid as a stabilizer. These QDs can be applied to the ultrasensitive detection of Pb2+ by virtue of their quenching effect. The size of the QDs exerts a strong effect on sensitivity, and the quenching of luminescence is most effective when the smallest particles are used. The detection limit is 4.7?nmol?L?1 when QDs-I (1.6?nm) are used, which is the lowest in the current related study.