The electrical resistivity of Al‐0.10 at% Ag alloy and pure aluminum is measured between room temperature and 500 °C in order to determine the phase diagram at low silver concentrations. The resistivity difference Δϱ between the alloy and pure aluminum changed corresponding to the phase diagram which includes GP zone formation. The solubility limit of silver in aluminum at 110 °C is 0.10 at% and the GP zone solvus temperature in the alloy is 90 °C. An abnormal increase in electrical resistivity is observed at 80 °C during the step‐cooling process. It seems to be caused by a precursory phenomenon of GP zone formation.