Accurately measuring the size distribution of ultrafine particles in the exhaust gas of motor vehicles is of great significance for air pollution and human health risk assessment. Based on previously developed unipolar charger, plate differential mobility analyzer (PDMA) and Faraday cup electrometer (FCE) module, a portable ultrafine particle sizer was developed, which can monitor size distribution of ultrafine particles from 20 to 500 nm in the exhaust gas of motor vehicles in real time. The calibration experiment of the self-developed particle size spectrometer was carried out. It is shown that the particle size peak measured by the portable motor vehicle ultrafine particle sizer is within ± 5% of the 40 nm and 100 nm single particle size selected by TSI 3081A/3082 and the difference between the number concentration measured by the sizer and that measured by Pegasor particle sensor (PPS-M) is ± 10%. Furthermore, field experiments were carriedout on different types of vehicles. The results show that the concentration of particulate matter in the exhaust gas of gasoline and diesel vehicles is mainly concentrated in the range of particle size less than 40 nm. In addition, in the contrast experiment, the median diameter of the peak measured by the self-developed particle sizer is 20 nm, and the median diameter measured by the commercial electrical low pressure impactor (ELPI) is 17 nm. The difference between the two particle sizers is 15%, indicating that the two particles sizers have good consistency.