The procedure followed by the Nuclear Metrology Laboratory (LMN) at the IPEN-CNEN/SP, in S~ao Paulo, for the primary standardization of 166Ho is described. The activity of 166Ho was determined by the efficiency extrapolation technique applied to a 4????(PC)-?? coincidence system using a gas flow proportional counter in 4?? geometry coupled to a 76 x 76 mm NaI(Tl) crystal. The results for the ??-rays intensities at 80.57 and 1379.45 keV were 0.0651(11) and 0.00904(11), respectively.Conselho Nacional de Desenvolvimento Cient??fico e Tecnol??gico (CNPq)CNPq: 306638/2017-7; 304730/2018-1