To investigate the transfer characteristics of PM2.5 in a car cabin, several experiments were performed featuring real-time online monitoring of the concentration of PM2.5 and CO2 in the cabin under different ventilation modes (circulation with outdoor air and recirculation without outdoor air), at a minimum of mechanical ventilation and matching experimental route conditions. We analyzed the PM2.5 concentration distribution characteristics and the ratio of inside-to-outside PM2.5 concentration (I/O). The ventilation rate was determined by the CO2 concentration change during the experiments, and it was used to analyze the transfer characteristics of PM2.5. The results showed that under the "circulation with outdoor air" condition, the average I/O was 0.6, while it was 0.25 under the "recirculation" condition. I/O value decreased with acceleration of driving speed. It could be concluded that the ventilation mode has a significant impact on the concentration of PM2.5 in the cabin, and plays a decisive role in PM2.5 levels. The quantitative evaluation of PM2.5 transfer characteristics under the "circulation with outdoor air" condition showed that the contributions of ventilation, penetration, and deposition to the cabin PM2.5 concentration accounted for 69.3%, 22.8%, and 7.9%, respectively; while under the "recirculation" condition, penetration accounted for 72.0% and deposition accounted for 28.0%. (C) 2016 Elsevier B.V. All rights reserved.