The yield strength and electrical conductivity of precipitation-hardenable Cu-2.05mass%Ni-0.35mass%Be and Cu-1.27mass%Ni-0.22mass%Be alloys aged at 360 to 560°C after cold rolling have been investigated. The former alloy exhibits a higher strength and a lower electrical conductivity in the under-aging and peak-aging stages, but a lower strength and a higher electrical conductivity in the over-aging stage than the latter alloy. Both alloys are hardened by plate-shaped coherent precipitates of γ" phase, which is body-centered tetragonal with a = b = 0.24nm and c = 0.28nm. In the peak-aging stage, the yield strength of both alloys is controlled by the precipitate shearing mechanism. Both alloys recrystallize in the over-aging stage at the coarse γ precipitates which are residual even after solutionizing. A higher number density of the coarse γ precipitates in the Cu-2.05%Ni-0.35%Be alloy gives rise to a larger volume fraction of recrystallization, resulting in a greater decrease in strength. The growth of incoherent precipitates in recrystallized grains is faster than that of coherent γ" precipitates in unrecrystallized grains. This causes a higher electrical conductivity of the Cu-2.05%Ni-0.35%Be alloy in the over-aging stage.