The　size　effect　has　been　the　major　subject　in　the　research　of　micro　forming.　For　the　phenomena　of　the　stress　change　in　the　metal　hydraulic　bulge　experiment　could　not　be　explained　properly　by　the　classical　plasticity　theory,　a　new　constitutive　mode　based　on　the　strain　gradient　theory　is　built　to　study　the　size　effect.　Based　on　the　model,　the　influences　of　the　thickness　of　metal　sheet　and　the　die　diameter　on　the　stress　in　hydraulic　bulge　experiment　is　studied,　the　stress　under　the　second　order　size　effect　can　be　described　well　by　theoretical　analysis　basing　on　the　model.　And　its　rationality　is　testified.