In　order　to　determine　how　large　vehicles　influence　the　traffic　flow,　traffic　volumes,　traffic　proportion　and　crashes　from　2002　to　2005　are　collected　from　Jingjintan　freeway　for　different　vehicle　categories.　The　whole　freeway　was　divided　into　sixty-four　sub-sections　according　to　their　specific　characteristics　andfurther　analysis　was　made　after　clustering　traffic　data　in　one　hour　interval.　The　percentage　of　large　vehicles　during　investigation　time　varied　from　21%　to　77%.　Bivariate　test　was　used　to　identify　factors　which　was　relate　to　large　vehicles　firstly.　It　is　found　that　speed　of　most　large　vehicles　is　40-50　km/h.　The　impacts　of　large　vehicles　on　speed　and　safety　were　also　figured　out.　There　are　more　large　vehicles　in　traffic　flow　in　midnight　than　other　times.　In　daytime,　proportion　of　large　vehicles　is　below　40%　and　this　number　goes　up　as　high　as　80%　and　reaches　peak　at　3:00　am.　The　speed　is　inversely　proportion　to　truck　percentage.　Linear　regression　technique　was　used　to　find　relationships　between　proportion　of　large　vehicles　and　traffic　flow　speed.　The　results　provide　a　method　that　how　should　speed　limit　be　set　according　to　large　vehicles　percentage.　However,　in　the　future,　more　work　have　to　be　done　to　examine　large　vehicles　affects　on　freeway　safety.　©　right.