Optical　Diffraction　Tomography　(ODT),　as　a　novel　3D　imaging　technique,　can　obtain　a　3D　refractive　index　(RI)　distribution　to　reveal　the　important　optical　properties　of　transparent　samples.　According　to　the　theory　of　ODT,　an　optical　diffraction　tomography　setup　is　built　based　on　the　Mach-Zehnder　interferometer.　The　propagation　direction　of　object　beam　is　controlled　by　a　2D　translation　stage,　and　121　holograms　based　on　different　illumination　angles　are　recorded　by　a　Charge-coupled　Device　(CCD).　In　order　to　prove　the　validity　and　accuracy　of　the　ODT,　the　3D　RI　profile　of　microsphere　with　a　known　RI　is　firstly　measured.　An　iterative　constraint　algorithm　is　employed　to　improve　the　imaging　accuracy　effectively.　The　3D　morphology　and　average　RI　of　the　microsphere　are　consistent　with　that　of　the　actual　situation,　and　the　RI　error　is　less　than　0.0033.　Then,　an　optical　element　fabricated　by　laser　with　a　non-uniform　RI　is　taken　as　the　sample.　Its　3D　RI　profile　is　obtained　by　the　optical　diffraction　tomography　system.