The　effect　of　Si　and　Sc　on　the　evolution　of　the　precipitates　during　aging　in　Al4.0Cu0.5　Mg　alloys　was　investigated　by　means　of　microhardness　tests　and　transmission　electron　microscope.　The　results　show　that　Si　can　significantly　refine　the　size　of　theta＇　phases,　because　addition　of　Si　introduces　a　considerable　amount　of　small,　uniformly　dispersed　Al-Cu-Mg-Si　(Q)　quaternary　phases,　which　provides　nucleation　sites　for　the　theta＇　phases.　Sc　atoms　segregate　at　the　interfaces　between　the　matrix　and　the　theta＇-Al2Cu　precipitates　in　AlCuMgSiSc　alloy.　The　segregation　of　Sc　atoms　inhibits　the　growth　of　the　theta＇　phases　and　improves　the　thermal　stability　of　the　theta＇　phases.　At　the　same　time,　the　segregation　of　Sc　atoms　in　Q　phases　restrains　the　coarsening　of　the　Q　phases,　which　also　contributes　to　the　thermal　stability　of　the　theta＇　phases.　The　co-addition　of　Si　and　Sc　thus　leads　to　a　combination　of　high　strength　and　high　thermal　stability　of　the　Al-Cu-Mg-Si-Sc　alloy.