The　aging　behavior　and　the　strengthening　mechanism　of　the　Si　microalloyed　Al-Zn-Mg-Cu　alloy　was　investigated　by　means　of　microhardness　tests,　tensile　properties　tests　and　transmission　electron　microscope.　It　was　found　that　the　addition　of　Si　element　enhanced　the　microhardness　and　strength　of　Al-Zn-Mg-Cu　alloy　after　aging　at　125　degrees　C,　175　degrees　C　and　225　degrees　C.　The　age　hardening　effect　of　Al-Zn-Mg-Cu　alloy　gradually　deteriorates　with　the　increase　of　aging　temperature,　while　the　age　hardening　effect　of　Si-containing　Al-Zn-Mg-Cu　alloy　at　different　aging　temperature　are　stable.　TEM　results　shows　that　the　intragranular　microstructure　of　the　Si　microalloyed　alloy　contained　a　fine　dispersed,　lath-like　precipitate.　This　lath-like　precipitation　phase　significantly　improves　the　strength　of　the　alloy　and　exhibit　thermal　stability,　it　is　considered　to　be　the　GPB-II　zone.