Digital　holographic　diffraction　tomography　combines　digital　holography　with　optical　diffraction　tomography.　According　to　the　Fourier　diffraction　theory,　the　spectrum　information　is　unevenly　distributed　on　a　Ewald　sphere,　and　most　of　these　data　cannot　exactly　locate　on　the　3D　matrix　points.　To　solve　this　problem,　a　single　assignment　based　nearest　neighbor　interpolation　method　is　proposed.　Firstly,　the　points　to　be　interpolated　are　chosen　on　the　3D　matrix.　For　each　angle,　a　search　scope　is　confirmed　by　two　spheres　with　a　radius　R　(k(0)-0.5＜　R　＜k(0)+0.5),　where　k0　is　the　radius　of　Ewald　sphere.　Then,　the　point　on　the　3D　matrix　is　assigned　by　the　value　of　the　nearest　neighbor　point　within　this　scope.　After　the　assignment　of　the　frequency　information　for　all　the　angles,　the　object　function　is　obtained　by　3D　inverse　Fourier　transform.　In　order　to　verify　the　feasibility　of　this　method,　a　digital　holographic　diffraction　tomography　system　is　built.　The　3D　refractive　index　(　RI)　distribution　of　a　microsphere　with　known　RI　1.4607　is　measured.　Comparing　with　the　conventional　nearest　neighbor　interpolation　algorithm,　the　relative　error　is　reduced　from　0.51%　to　0.36%.　It　is　demonstrated　that　the　proposed　algorithm　can　improve　the　reconstruction　accuracy　for　diffraction　tomography.