Aging process in Al-Zn alloys was studied by the measurements of the electrical resistivity when the alloy was quenched from a high temperature, about 300℃ (the first quenching temperature), to an intermediate temperature, between 110℃ and 230℃ (the second quenching temperature), held at this temperature for a time, quenched again into iced water and aged at a low temperature. Variation of the holding time at the second quenching temperature brings about the variation of the isothermal aging curves. Maximum resistivity of the isothermal aging curve, p(max) , decreases at first, passes a minimum and then increases to reach a stationary value as the holding time at the intermediate temperature increases. It is pointed out that this phenomenon is mainly due to the fluctuation of solute concentration and the vacancy concentration decreasing at the intermediate temperature.