Ultra-fine particles emitted by motor vehicles which have high number concentration and surface area, are one of the main sources of air pollution,. Accurate on-line monitoring is of significance for vehicle emission assessment and control measures. With the addition of particle number (PN) control index in vehicle emission standards in EU and China, the requirements of on-line monitoring technology are also increasing. At present, diffusion charging method and condensation nuclear particle count method are mainly used to measure the number of ultra-fine particles emitted by motor vehicles, and aerodynamics and electro-migration classification methods are mainly used to measure the particle size distribution. The research status of on-line monitoring technology of ultra-fine particles emitted by motor vehicles at home and abroad is summarized, the principle, characteristics and application status of several main developed measurement technologies and instruments are mainly introduced, and the future development of the technologies is prospected.