A　practical　3D　positioning　system　was　developed　based　on　the　mathematical　models　of　3D　magnetic　field　induced　by　a　single　loop　coil.　Mathematical　expression　of　the　magnetic　intensity　in　3D　space　was　derived　in　detail.　3D　positioning　using　magnetic　field　provides　a　new　approach　to　detect　the　location　of　a　non-visible　target　inside　a　non-magnet-shielded　object　such　as　human　body　by　taking　advantage　of　penetrative　capability　of　magnetic　field　through　such　object.　By　attaching　magnetic　sensor　to　medical　instruments,　the　signals　generated　by　the　sensor　inside　the　body　response　to　the　magnetic　field　produced　by　emitter　coils　outside　body　are　analyzed　to　determine　the　location　of　sensor　as　well　as　the　position　of　the　medical　instrument.　Real-time　six-degree　freedom　measurement　for　3D　magnetic　sensor　and　five-degree　freedom　measurement　for　single　magnetic　sensor　were　realized.　This　system　provides　a　new　approach　to　determine　the　3D　position　and　orientation　of　endoscope　inside　the　patient＇s　body　by　harmless　magnetic　field.