We performed fluorescence microscopic observation of the conformational change of individual T4DNAs (166 kbp) induced by Fe2+/Fe3+. Individual DNAs undergo a marked discrete transition from an elongated coil into a collapsed globule with an increase in the Fe3+ concentration at around 1-2 mu M. On the other hand, DNAs remained in the elongated coil state with the addition of Fe2+ up to a concentration of 30 mu M. Using these experimental results we tried to control the transition of DNA by the redox reaction of Fe2+/Fe3+. We found that collapsed globule DNA unfolds with the reduction of Fe3+. The results have been analyzed theoretically in terms of the double minima in the free energy profile of a single DNA molecule, indicating that the change in the translational entropy of the counterions is the main reason why high-valency ions are more effective in inducing the collapse.