Two　way　satellite　time　and　frequency　transfer　(TWSTFT)　is　the　most　precise　remote　time　transfer　method.　The　comparison　precision　of　TWSTFT　could　approximately　reach　0.3ns.　The　basic　principle　and　construction　of　TWSTFT　in　National　Institute　Metrology　(NIM)　were　demonstrated.　When　the　data　was　processing,　software　standardization　guide　made　by　Consultative　Committee　for　Time　and　Frequency　(CCTF)　was　adopted　and　the　comparison　precision　of　NIM　is　about　0.35ns.　According　to　the　TWSTFT　schematic　diagram　of　transfer　path,　equipment　delay,　ionosphere　refraction　and　satellite　motion　relative　to　ground　station　are　the　main　error　sources　to　the　comparison　precision.　The　results　show　that　the　equipment　delay　error　is　the　major　source　in　the　TWSTFT　with　the　value　of　0.12ns.　©　2011　IEEE.