The procedure followed by the Nuclear Metrology Laboratory (LMN) at the IPEN for the standardization of activity of 64Cu is described. The measurement was carried out in a 4?? (PC) ??-?? coincidence system. The activity per unit mass of the solution was determined by the extrapolation technique. The events were registered using a Time to Amplitude Converter (TAC) associated with a Multi-channel Analyzer. The gamma-ray emission probability per decay of the 1345.7 keV transition of 64Cu determined with a calibrated REGe gamma-ray spectrometer was (0.472??0.010) %, in agreement with the literature.