Vehicle　emission　factors　(EFs)　are　important　factors　affecting　tunnel　environments.　In　this　study,　field　tests　were　conducted　in　an　expressway　tunnel　in　southwest　China.　The　percentage　of　heavyduty　vehicles　(HDVs)　was　approximately　38　%.　The　NOx　concentrations　exceeded　the　safety　thresholds.　The　average　CO,　NOx,　and　PM2.5　EFs　of　the　mixed　fleet　were　1.25,　2.68,　and　0.079　g/　km　veh-　1,　respectively.　The　NOx　and　PM2.5　EFs　of　the　HDV　were　31.3　and　14.7　times　higher　than　those　of　the　light-duty　vehicle　(LDV),　respectively.　HDVs　contributed　61.5　%　to　CO,　94.8　%　to　NOx,　and　89.3　%　to　PM2.5　emissions　in　this　investigation.　When　compared　to　previous　studies,　a　significant　reduction　in　CO　and　PM　was　observed,　but　NOx　remained　at　a　high　level.　Stricter　policies　are　needed　to　control　the　emissions　of　HDVs,　and　the　ventilation　of　tunnels　must　consider　the　NOx　concentration.