Flow,　speed,　and　density　are　three　basic　variables　that　can　be　used　to　describe　the　traffic　flow　characteristics.　The　relationship　among　them　is　the　foundation　of　the　traffic　engineering　research.　This　paper　started　at　the　basic　character　of　the　traffic　flow,　deduced　the　theoretical　formulas　of　the　relationship　between　traffic　density　and　vehicle　speed　according　to　the　three　phases,　and　further　discussed　the　theoretical　formulas　of　the　relationship　between　traffic　volume　and　vehicle　speed,　traffic　volume　and　traffic　flow　density.　The　formulas　accord　with　the　curves　of　field　survey　results.