The aging of Cu–Ni–Si–Zn alloy for lead frame is investigated. The results showed that the peak of hardening effect occurs after aging for about 1 h and the electrical conductivity increases continuously with aging times. The hardness of the alloy reached a peak at 430–460 °C for 2 h and electrical conductivity reached a peak at 500–550 °C and continuously decreased afterwards. The cold rolling prior to the aging treatment was used to increase the precipitation rate. The precipitates responsible for the age-hardening effect are disc-shaped δ-Ni 2 Si, which has an orthorhombic structure.