The MEG experiment makes use of one of the world's most intense low energy muon beams, in order to search for the lepton flavour violating process mu(+) -> e(+)gamma. We determined the residual beam polarization at the thin stopping target, by measuring the asymmetry of the angular distribution of Michel decay positrons as a function of energy. The initial muon beam polarization at the production is predicted to be P-mu = -1 by the StandardModel (SM) with massless neutrinos. We estimated our residual muon polarization to be P-mu = -0.86 +/- 0.02 (stat)(+0.05)(-0.06) (syst) at the stopping target, which is consistent with the SM predictions when the depolarizing effects occurring during the muon production, propagation and moderation in the target are taken into account. The knowledge of beam polarization is of fundamental importance in order to model the background of our mu(+) -> e(+)gamma search induced by themuon radiative decay: mu(+)-> e(+)(nu) over bar (mu)nu(e)gamma.