Abstract: The effects of Sn on microstructure and property of Al-Zn-Mg-Cu alloy after aging at 120 ℃ for different times were studied by transmission electron microscopy and vickers microhardness measuring. The results show that Sn could promote forming GPII zones at the early stage of aging. The high vacancy binding energy of Sn in the alloy containing Sn could affect element′s diffusing, thus restrain the coarsening of η′ phase and delay the hardness peak, and the alloy containing Sn had higher hardness.