The precipitation process in 17-4 PH stainless steel was investigated by measuring the changes in electrical resistivity at the liquid nitrogen temperature. From the results of derivatves of resistivity changes obtained by isochronal aging, three kinds of precipitation reactions P1, P2 and P3, tentatively named, were detected at about 410℃, 450℃ and 650℃ respectively. The activation energy for P1 precipitation reaction determined from resistivity measurement was about 49㎉/㏖ and this P1 precipitation was interpreted as the formation of Ni-rich zone. P1 was detected more evidently during re-aging at low temperature after high temperature aging at 560℃. The activation energy for P2 reaction was about 53㎉/㏖ and P2 was interpreted as the formation of Cu-rich phase.