The Observable is calculated as a function of the di-lepton invariant mass. The forward backward asymmetry has been calculated at detector level and then corrected with an unfolding technique. Two corrections are applied: detector resolution and QED radiative corrections (namely mass bin migration), dilution. The results are presented for the cases when only the mass bin migration correction has been applied and when both the mass bin migration and dilution corrections have been applied. A measurements of $A_{FB}$ at Dressed level is also shown. The forward-backward asymmetry has been calculated for three different final states: electron Central-Central (CC), electron Central-Forward (CF) and muons. CC electrons are required to have $p_{T}>25$ GeV and $|\eta|<2.4$, CF electrons are required to have $p_{T}>25$ GeV, one electron with $|\eta|<2.4$ and one electron with $2.5<|\eta|<4.9$. Muons are required to have $p_{T}>20$ GeV and $|\eta|<2.4$. The $A_{FB}$ has been calculated for di-lepton invariant-masses $66<m_{ll}<1000$ GeV for electrons CC and muons and $66<m_{ll}<250$ GeV for electrons CF. Data have been collected by the ATLAS detector in 2011, corresponding to an integrated luminosity of 4.8 fb$^{-1}$.