Based　on　the　point　spread　function　of　holographic　system,　the　lateral　resolution　of　digital　holographic　imaging　system　without　any　pre-magnification　is　studied.　The　expression　of　resolution　limitation　of　holographic　imaging　system　is　thus　presented.　We　investigate　the　possibilities　to　improve　the　lateral　resolution.　The　simple　experimental　setup　with　an　off-axis　arrangement　is　built.　By　using　a　U.S.　Air　Force　(USAF)　test　target　as　microscopic　object,　the　recorded　holograms　are　reconstructed　digitally　based　on　the　principle　of　Fresnel　diffraction.　The　lateral　resolution　of　2.76　mu　m　without　any　pre-magnification　is　demonstrated　experimentally,　which　matches　the　theoretical　prediction　well.