Five sizes of water-dispersed CdTe quantum dots (QDs) stabilized by thioglycolic acid (TGA) with a high photoluminescence (PL) quantum yield were synthesized and a size dependent quenching of the fluorescence by cobalt ions was also observed. No matter for smaller or larger particles, obvious quenching effect was observed, and the fluorescence quenching of CdTe nanoparticles depended on the concentration of cobalt ions solution. However, CdTe QDs with different size showed dramatically different quenching efficiency, sensitivity, linear range and selectivity. With the increase of size, the quenching efficiency reduced correspondingly. The smallest particle was the most sensitive with the limit of detection for cobalt ions is 7.3 x 10(-9) molL(-1) Co(2+). For larger particles, the sensitivity was much lower, but the linear range was relatively wide, under optimal conditions, the quenched fluorescence intensity increased linearly with the concentration of cobalt ions ranging from 3.32 x 10(-8) to 3.62 x 10(-6) molL(-1). Besides, the influence on the fluorescence signal of foreign cations, including Ca(2+), Mg(2+), Ni(2+), Ba(2+), Zn(2+), Cu(2+), Fe(3+) and Ag(+) were also studied, results showed a high selectivity of the smaller QDs towards cobalt ions. According to Stern-Volmer-type equation, quenching of quantum dot luminescence was most effective for the smallest particles with the highest K(sv).