Ultrafine particles originating from vehicle exhausts may deteriorate the air quality inside the street canyon, so it is of importance for the air quality control to clarify the formation and dispersion characteristics of ultrafine particles. In this study, the population balance model considering the nucleation, condensation and evaporation, as well as the species transport and RNG k-epsilon turbulent models were employed, and the sensitivities of the dispersion of ultrafine particles to the wind direction, emission rates of H2SO4 vapor and organic compounds, traffic flow and ambient temperature inside an isolated street canyon were analyzed. The results show that the wind direction is a key issue to the particle number concentration. The emission rates of H2SO4 vapor and organic compounds play dominant roles in the formation and growth of particles, so should be strictly controlled to reduce the air pollution. Restricting the use of private vehicles based on the even-and-odd number licensed trip can conspicuously lower the ultrafine particle concentrations by more than one order of magnitude. More powerful nucleation among the vehicle exhausts will be triggered in a cold environment, therefore the haze pollution may break out in winter more easily. (C) 2016 Elsevier B.V. All rights reserved.