This　paper　presents　a　study　of　the　critical　gap　of　return　headway　on　two-lane　highway,　which　could　work　as　the　criterion　to　judge　if　the　vehicle　is　in　the　car-following　state.　The　following　headway　under　the　car-following　state　acquired　by　the　critical　gap　of　return　headway　offers　a　strong　support　to　the　research　of　two-lane　highway　capacity　with　qualitative　and　quantitative　analysis.　The　paper　provides　a　measurement　to　get　the　value　of　return　headway　by　passing　experiment.　Logit　model　is　used　to　calculate　the　critical　gap　of　return　headway　according　to　the　data　collected　from　passing　experiment.　Copyright　ASCE　2007.