The regional transport of PM_2.5 plays an important role in the air quality over the Beijing-Tianjin-Hebei (BTH) region in China. However, previous studies on regional transport of PM_2.5 are mainly at province level, which is insufficient for the development of an optimal joint PM_2.5 control strategy. In this study, we calculate PM_2.5 inflows and outflows through the administrative boundaries of three major cities in the BTH region, i.e. Beijing, Tianjin and Shijiazhuang, using the WRF (Weather Research and Forecasting model) – CMAQ (Community Multiscale Air Quality) modelling system. The monthly average inflow fluxes indicate the major directions of PM_2.5 transport. For Beijing, the PM_2.5 inflow fluxes from Zhangjiakou (on the northwest) and Baoding (on the southwest) constitute 57 % of the total in winter, and Langfang (on the southeast) and Baoding constitute 73 % in summer. Based on the net PM_2.5 fluxes and their vertical distributions, we find there are three major transport pathways in the BTH region: the Northwest-Southeast pathway in winter (at all levels below 1000 m), the Southeast-Northwest pathway in summer (at all levels below 1000 m), and the Southwest-Northeast pathway both in winter and in summer (mainly at 400–1000 m). In winter, even if surface wind speeds are small, the transport at above 400 m could still be strong. Among the three pathways, the Southwest-Northeast happens along with PM_2.5 concentrations 30 % and 55 % higher than the monthly average in winter and summer, respectively. Analysis of two heavy pollution episodes in January and July in Beijing show a much stronger (8–16 times) transport than the monthly average, emphasizing the joint air pollution control of the cities located on the transport pathways, especially during heavy pollution episodes.