Water-soluble CdTe quantum dots (QDs) with five sizes (2.25, 2.50, 2.77, 3.12, and 3.26nm) were synthesized with the hydrothermal method. The electrochemiluminescence (ECL) of CdTe QDs was investigated in detail in air-saturated solution without adding foreign oxidant. It was found that the ECL of CdTe QDs displayed a size-dependent property. With the increasing in the particle size of the CdTe QDs, the ECL intensity was gradually increased, in addition, both ECL peak potentials and ECL onset potentials of CdTe QDs were shifted positively. Influences of some factors on the ECL intensity were investigated. Under the optimal conditions, the ECL intensity had a linear relationship with the concentration of l-cysteine (l-Cys) in the range from 1.3 x 10(-6) to 3.5 x 10(-5) molL(-1) (R(2) 0.996) with a detection limit of 8.7 x 10(-7) molL(-1) (S/N=3). The proposed method was applied to the determination of l-Cys in real samples with satisfactory results. Compared with previous reports, it has better selectivity for the determination of l-Cys.