Terahertz　digital　holography　is　a　combination　of　terahertz　technology　and　digital　holography.　In　digital　holography,　the　imaging　resolution　is　the　key　parameter　in　determining　the　detailed　quality　of　a　reconstructed　wavefront.　In　this　paper,　the　synthetic　aperture　method　is　used　in　terahertz　digital　holography　and　the　in-line　arrangement　is　built　to　perform　the　detection.　The　resolved　capability　of　previous　terahertz　digital　holographic　systems　restricts　this　technique　to　meet　the　requirement　of　practical　detection.　In　contrast,　the　experimental　resolved　power　of　the　present　method　can　reach　125　mu　m,　which　is　the　best　resolution　of　terahertz　digital　holography　to　date.　Furthermore,　the　basic　detection　of　a　biological　specimen　is　conducted　to　show　the　practical　application.　In　all,　the　results　of　the　proposed　method　demonstrate　the　enhancement　of　experimental　imaging　resolution　and　that　the　amplitude　and　phase　distributions　of　the　fine　structure　of　samples　can　be　reconstructed　by　using　terahertz　digital　holography.　(C)　2015　Optical　Society　of　America