The　effect　of　T4,　T6,　T77　and　T73　ageing　treatments　on　microstructure　and　stress　corrosion　cracking　of　an　Al-4.2Zn-1.6Mg　alloy　was　investigated.　TEM　study　revealed　that　there　is　no　eta＇　phase　on　the　matrix　or　grain　boundary　in　the　T4　sample,　and　the　precipitates　on　the　grain　boundaries　of　the　T6　sample　were　small　and　continuously　distributed,　while　the　precipitates　of　the　T77　and　T73　samples　discontinuously　distributed　on　the　grain　boundary.　Stress　corrosion　cracking　resistance　test　in　in　the　50　degrees　C　3.5%　NaCl　solution　and　fractured　surface　observation　revealed　that　the　T4　sample　have　higher　tendency　of　stress　corrosion　cracking　compared　with　that　of　the　T6　sample,　while　the　T77　and　T73　samples　show　good　resistance　of　stress　corrosion　cracking.　It　is　worth　to　note　that　the　strength　of　the　T77　sample　is　very　close　to　that　of　the　T6　sample.