China strives to achieve carbon emission peaking by 2030,and the exhaust from vehicles on road are an important source of greenhouse gas (GHG) emissions.Due to the upgrading of vehicle emission limits and fuel standards,it is particularly necessary to study the GHG emission factors of currently in-use vehicles.In this study,14 in-use light- and heavy-duty vehicles in China were tested for CO₂ (carbon dioxide) Emission Factors (EFs) under the cold start procedure of WLTC (Worldwide harmonized Light-duty Test Cycle) and C-WTVC (China-World Transient Vehicle Cycle) operating conditions respectively using chassis dynamometer,and the corresponding fuel consumptions as well as influencing factors were studied.The results show that the CO₂ EFs of vehicles are affected by displacement,hot／cold start,fuel and driving condition.Vehicles have the highest fuel consumption under cold start conditions on urban roads,resulting in higher CO₂ EFs,which are 26.6％-199.7％ and 8.3％-35.5％ higher than those under full working cold start and hot start conditions on urban roads,respectively.Meanwhile,the higher fuel consumption of high-displacement heavy-duty diesel trucks in urban traffic condition leads to a significant increase in CO₂ EFs,so banning high-displacement heavy-duty diesel trucks from entering urban areas can effectively control CO₂ emissions.Using LPG (Liquefied Petroleum Gas) as a fuel substitute can reduce CO₂ emissions from vehicles.LPG buses and taxis would reduce CO₂ EFs by 37.2％ and 12.1％ on urban roads,and by 51.8％ and 20.3％ on highway roads.The current WLTC conditions,which are more in line with actual road conditions,still underestimate the CO₂ EFs and fuel consumption of light-duty vehicles on Chinese roads by 31％-46％ and 17.7％-26.8％,therefore to obtain more accurate vehicle emission data,we must accelerate the localization of vehicle test cycles.