Abstract: Two typical in-use diesel buses meeting "China Ⅳ" emission standard were selected to do on-road emission tests using a portable emission measurement system in Beijing.The road distance of testing was 51.3 km,the time of testing was 10645s,and 10645 data groups were obtained of the buses.The running cycle in the testing can typically reflect the real on-road running cycle.The emission factors of the tested buses are(11.02±1.46),(1.44±0.12),(0.038±0.01),and(0.053±0.03) g/km for NO_x,CO,total carbureted hydrogen(THC),and particulate matter(PM),respectively.Results show that fuel consumption and pollutant emission are strongly related to the speed and acceleration in the driving cycles.Higher pollutant emission rates and fuel consumption are tested during the high speed and acceleration zone in the driving cycles.Higher NOx emission rates are mainly during the high speed and acceleration zone.Higher CO emission rates are mainly during the vehicle speed exceeding 30 km/h or during the acceleration zone.Higher THC emission rates were mainly during acceleration zone while the vehicle speed was slower than 30km/h.