The electromagnetic calorimeter (ECAL) of the CMS detector contains about 76000 lead-tungstate (PbWO4) crystals. This calorimeter has an excellent intrinsic energy resolution, below 0.5% above 120 GeV. At high energies, the major contribution to the resolution is given by the inter-calibration of the channels that compose the detector. It is therefore important to keep this contribution below 0.5%. To reach such a precision, several techniques have been envisaged, both before and after the start of the experiment. During the data taking, in particular, physics channels will be used to equalize the response of all the PbWO4 crystals of the detector. At the very beginning the symmetry of the minimum bias deposition, together with the Z invariant mass reconstruction, will provide a first estimate of the calibration coefficients, that will be eventually determined by using the E/p ratio of isolated electrons, mainly coming from the W decay. The π 0 and η 0 decays will also help determining the calibration coefficients.