Actual　headway　oscillating　around　expected　headway　(oscillating　process)　is　a　theoretical　basis　of　steady-state　car-following　model.　To　explore　the　relationship　between　the　oscillation　amplitude　and　average　velocity　of　following　vehicle,　we　studied　the　real　traffic　flow　from　steady-state　car-following　on　Beijing　urban　expressway.　When　average　velocity　of　the　following　vehicle　changed　within　a　certain　range,　the　amplitude　described　a　piecewise　linear　function;　when　average　velocity　of　the　following　vehicle　was　less　than　the　lower　threshold　of　this　range,　the　amplitude　tended　to　0;　when　average　velocity　of　the　following　vehicle　was　greater　than　the　upper　threshold　of　the　range,　there　was　no　significant　correlation　between　the　amplitude　and　average　velocity　of　the　following　vehicle.　The　threshold　of　the　range　and　the　inflection　point　of　the　piecewise　function　might　relate　to　driving　environment,　vehicle　and　driver　attributes.　©　2011　ASCE.