The　stress　relaxation　behavior　of　age-forming　for　an　Al-Zn-Mg-Cu　alloy　was　studied　using　a　designed　device　that　can　simulate　the　age　forming　process.　The　mechanism　of　stress　relaxation　was　also　revealed　through　calculating　thermal　activation　parameters　and　analyzing　the　microstructures.　The　results　suggested　that　the　stress　relaxation　behavior　of　the　Al-Zn-Mg-Cu　alloy　in　the　simulated　age-forming　process　can　be　divided　into　three　stages　according　to　the　stress　level.　The　three　stages　of　stress　relaxation　are:　(i)　the　initial　high　stress　stage,　(ii)　the　subsequent　middle　stress　transition　stage　and　(iii)　the　last　low　stress　equilibrium　stage,　respectively.　The　deformation　activation　energies　are　132　kJ/mol　in　the　initial　high　stress　stage,　119　kJ/mol　in　the　subsequent　middle　stress　transition　stage　and　91　kJ/mol　in　the　last　low　stress　equilibrium　stage,　respectively.　The　analysis　of　the　thermal　activation　parameters　and　microstructures　revealed　that　dislocation　creep　was　the　dominant　deformation　mechanism　in　the　initial　and　subsequent　stages　of　the　stress　relaxation;　whereas　diffusion　creep　is　the　mechanism　in　the　last　stage　of　the　stress　relaxation.　Additionally,　a　special　threshold　stress　phenomena　was　present　in　the　stress　relaxation　of　the　age-forming　process,　which　was　scribed　to　the　interaction　between　precipitation　and　dislocation　in　the　Al-Zn-Mg-Cu　alloy　©　2013　Elsevier　B.V.