The　influence　of　Zn/Mg　ratio　on　microstructure　and　mechanical　properties　of　Al-Zn-Mg　alloys　with　the　same　Zn　+　Mg　contents　has　been　investigated　through　microhardness　test,　electrical　conductivity　test　and　transmission　electron　microscopy　(TEM).　In　this　study,　Zn/Mg　ratio　was　changed　from　1.61　to　3.93　with　Zn　+　Mg　content　of　6　wt%.　The　results　showed　that　the　Vickers　hardness　of　the　alloy　with　the　Zn/Mg　ratio　of　2.38　aged　at　120　degrees　C　for　48　h　was　highest.　Through　TEM　observation,　strengthening　precipitates　in　investigated　alloys　were　mainly　spherical　T＇　phases　and　a　few　plate-like　eta＇　phases.　With　the　increase　of　Zn/Mg　ratio,　T＇　phase　fraction　in　alloys　decreased　from　88%　to　81%,　and　the　corresponding　eta＇　phase　fraction　increased　from　12%　to　19%.　As　aging　at　120　degrees　C　from　48　h　to　168　h,　the　coarsening　degree　of　precipitates　of　alloy　A　and　alloy　C　were　both　greater　than　that　of　alloy　B.　When　the　Zn　+　Mg　content　was　6　wt%,　the　alloy　with　the　Zn/Mg　ratio　of　2.38　had　optimum　microstructure　and　mechanical　properties.