The　precipitation　behaviors　and　thermal　stability　of　Al-3.5Mg-1.0Cu　alloys　with　co-addition　of　Zn　and　Si　were　investigated　in　this　study.　The　results　show　that　the　addition　of　Zn　to　Al-Mg-Cu　alloys　enhances　the　age-hardening　response　because　of　the　formation　of　T-Mg32(Al,　Zn)49　type　phase,　and　the　addition　of　Si　further　increases　the　peak　hardness　by　refining　the　size　of　cube-like　T　phase.　The　majority　of　the　cube-like　T　phases　show　the　excellent　thermal　stability　when　aging　at　175　degrees　C　while　some　T　phases　with　low　Cu　content　are　coarsen　seriously　and　transform　to　the　lath-like　shape.　The　addition　of　Si　also　enhances　the　thermal　stability　by　narrowing　the　size　distribution　of　precipitates.　Two-step　aging　treatment　leads　a　higher　peak　hardness　but　a　deteriorative　thermal　stability.(c)　2023　Elsevier　B.V.　All　rights　reserved.