This　work　is　aimed　to　optimize　the　heat　treatment　parameters　of　Ly12　(Al　-Cu　-Mg)　alloy　for　improvement　in　properties.　Also　to　study　the　aging　behavior　for　determination　of　peak　strength　and　highlighting　the　limitations　of　over　-aging　(over　-burning).　Normal　composition　of　the　alloy　is　4.0　Cu,　0.7　Mn,　0.8　Mg,　0.5　Si　and　balance　Al.　Thermal　treatment　process　to　achieve　the　ultimate　properties　is　solution　treatment　at　500　C　followed　by　quenching　in　cold　water　and　then　natural　aging　at　room　temperature　for　96　hours.　Main　phases　in　this　alloy　are　Mg2Si　&　CuAl2.　If　the　heating　temperature　goes　beyond　500　C,　the　over　-aging　of　Mg2Si　phase　occurs.　Over　-aging　is　grouped　into　three　categories　a)　slightly　over　-ageing　b)　medium　over　-ageing　and　c)　serious　over　-ageing.　In　the　present　work　samples　of　Al　alloy　were　heat　treated　at　different　temperatures　and　then　the　effect　of　over　burning　on　microstructure　and　mechanical　properties　was　studied.