The　effect　of　different　size　precipitates　on　stress　relaxation　behavior　of　7050　aluminum　alloy　during　age-forming　process　was　investigated,　which　is　helpful　to　apply　creep　age-forming　technology　of　7xxx　series　aluminum　alloy　with　different　tempers.　The　results　show　that　the　precipitates　can　obstruct　dislocation　thermal　activation,　and　therefore　7050　aluminum　alloy　with　different　sizes　of　precipitates　obviously　exhibits　different　stress　relaxation　behaviors,　and　relaxation　rate　decreases　and　relaxation　limit　increases　with　increasing　precipitates　size.　The　calculation　of　dislocation　activation　volume　and　microstructure　characterization　proves　that　the　increasing　precipitates　size　has　a　significant　obstruction　effect　on　dislocation　motion　in　stress　relaxation　process.　Besides,　a　special　threshold　stress　phenomenon　is　present　in　the　stress　relaxation　of　the　age-forming　process,　and　the　threshold　stress　increases　with　the　increase　of　precipitates　size.