  Based on a sample of 225.3 million J/\psi events accumulated with the BESIII
detector at the BEPCII, the decays of \eta' to pi+pi-l+l- are studied via
J/\psi to \gamma\eta'. A clear \eta' signal is observed in the pi+pi-e+e- mass
spectrum, and the branching fraction is measured to be \BR(\eta' to pi+pi-e+e-)
= (2.11\pm0.12 (stat.)\pm0.15 (syst.))\times10^{-3}, which is in good agreement
with theoretical predictions and the previous measurement, but is determined
with much higher precision. No \eta' signal is found in the pi+ pi- mu+ mu-
mass spectrum, and the upper limit is determined to be \BR(\eta' to pi+ pi- mu+
mu-)<2.9\times10^{-5} at the 90% confidence level.
