Although AlMgSi (6000－series) are generally considered to have better corrosion resistance than other aluminum alloys, it may introduce susceptibility to intergranular corrosion (IGC) by some factors, for instance, improper thermo－mechanical treatment and high Cu content. So the T78 treatment has been designed to desensitize it to IGC. In this paper, effects of T6 and T78 tempers on the microstructures and properties of an Al－0.75Mg－0.75Si－0.8Cu (mass fraction, %) were investigated by hardness test, electric conductivity test,accelerated corrosion test, SEM, TEM and energy dispersive X－ray elemental mapping. The hardness and conductivity of the T6 peak－aged sample, which was obtained by artificial aging for 8 h at 180 ℃, were 128.3 HV and 27.3~10⁶ S/m, respectively. T78 tempers involved a first step aging (180 ℃, 5 h) followed by a second step aging at 195, 205 and 215 ℃, respectively.With the prolonging of second step aging, the hardness firstly decreased then increased, while the conductivity increased gradually. The optimum T78 process was(180 ℃, 5 h)+(195 ℃, 2 h), at which condition the hardness was 129.2 HV and the electric conductivity was 27.6~10⁶ S/m. TEM observation results show that it was mostly needle－likeβ” phase in the Al matrix for the peak－aged sample. After T78 treatment, a large amount of lath－like precipitates formed in the matrix, while the precipitate free zones (PFZ) broadened slightly. The segregation of Cu was found at the interface between lath－like precipitates and the matrix, so more Cu precipitated out from the Al matrix and thus reduced the electrochemical potential difference between the PFZ and the matrix. The above finding may explain why T78 temper can desensitize intergranular corrosion without sacrificing strength.