Abstract The interaction between dsDNA and CdTe QDs was investigated by an indirect electrochemical method with the assistance of electroactive dsDNA indicator Co ( phen ) 3 3 + / 2 (phen = 1,10-phenanthroline). The results indicated that CdTe QDs had an obvious effect on Co ( phen ) 3 3 + / 2 + /dsDNA system. Dissociation method was employed for eliminating the influence of CdTe QDs on the signal from Co ( phen ) 3 3 + / 2 + . It was found that Co ( phen ) 3 3 + / 2 + was more easily dissociated from dsDNA modified gold electrode (dsDNA/Au) in the presence of CdTe QDs. In relatively low ionic strength Tris buffer (pH 7. 0, 5 mM NaCl), the dissociation coefficient constant of Co ( phen ) 3 3 + / 2 + in the presence of CdTe QDs was 3.1 times larger than that in the absence of CdTe QDs, while it was 1.32 times in relatively high ionic strength Tris buffer (pH 7.0, 50 mM NaCl), indicating that the binding site of CdTe QDs on dsDNA was probably at major groove of dsDNA. This demonstration offers a new approach to illustrate the QDs cytotoxicity mechanism.