Microstructure　of　commercial　5052　aluminum　alloy　subjected　to　repetitive　groove　pressing,　using　a　novel　self-designed　groove　pressing　die,　was　investigated　by　optical　microscopy　and　transmission　electron　microscopy.　The　results　show　that　grain　refinement　has　been　achieved　using　constrained　groove　pressing　(CGP)　or　unconstrained　groove　pressing　(UGP).　The　average　grain　size　gets　decreased　with　the　increasing　of　the　number　of　pressing.　But　the　microstructure　of　the　samples　processed　by　different　groove　pressing　methods　exhibits　different　characteristics.　The　samples　processed　by　CGP　exhibit　equiaxed　ultra-fine　grains　and　subgrains　with　high　dislocation　density　inside.　The　microstructure　of　the　samples　subjected　to　UGP　can　be　characterized　as　elongated　grains　and　a　uniform　distribution　of　dense　dislocation　microbands　which　have　misorientation　from　one　region　to　another.　Compared　with　UGP,　CGP　is　more　beneficial　to　grain　refinement　and　formation　of　equiaxed　grains.