The electromagnetic calorimeter (ECAL) of the CMS experiment, that will take data at LHC, has been designed to get an excellent energy resolution, essential to search for diphotonic resonances. ECAL is made of lead tungstate
crystals whose individual response, that depends on several contributions, affects the resolution of the Whole detector. Hence, a channel-to-channel intercalibration
is required to get the goal performances. The calibration of the detector at the start-up and the in situ calibration strategies are reported
