The　size　effect　is　the　major　subject　in　the　research　on　micro　forming　of　metal　sheet.　In　this　paper,　tensile　tests　of　brass　sheet　with　various　thicknesses　and　widths　were　performed　to　investigate　the　size　effect　on　the　flow　stress　and　plastic　properties.　The　results　showed　that　the　flow　stress　was　sensitive　to　the　brass　sheet　thickness.　The　flow　stress　was　hardened　by　the　geometrically　necessary　dislocation　that　continually　increased　with　the　inhomogeneous　deformation　when　the　brass　sheet　thickness　decreased　to　the　level　of　10　times　of　the　characteristic　length.　The　yield　strength　and　tensile　strength　increased　with　the　decrement　of　thickness.　But　the　hardening　effect　of　the　geometrically　necessary　dislocation　density　on　the　flow　stress　was　weakened　because　of　the　decrement　of　the　ratio　of　the　geometrically　necessary　dislocation　density　to　the　statistically　stored　dislocation　density.　The　fracture　surface　analysis　of　tensile　specimen　revealed　the　size　effect　mechanism　of　elongation.　The　elongation　of　the　brass　sheet　with　thickness　less　than　50　μm　was　lower　because　its　fracture　mechanics　was　intergranular　fracture.　As　the　fracture　mechanics　was　dimple　fracture　and　the　holes　postponed　the　fracture,　the　elongation　of　the　thicker　brass　sheet　was　good.