Air pollution is an important environmental and health concern all over the world and PM is one of the most important constituents of air pollution. In urban area with high population density, vehicles contribute a big portion of PM. The effect of vegetations along road, i.e., road greenbelts, on PM concentration is still a hot research topic. This study used three-dimensional green volume (3DGV, the three-dimensional volume of the crown and stems of all vegetations including trees, shrubs and grass) to evaluate the vegetation quantity of road greenbelts along four main roads in Nanjing, China. High spatial resolution images were collected with unmanned aerial vehicle (UAV) for othomosaic and feature extraction analysis. A Geographic Information System (GIS) database was developed to cover the location, crown diameter, crown height, and 3DGV information of vegetations in the road greenbelts. The environmental benefits of the road greenbelts were evaluated based on 3DGV information. The relationship between 3DGV of road greenbelts and PM concentration was analyzed and it was found that large 3DGV does not mean lower PM concentration. A road greenbelt with even vertical distribution of biomass and diversified vegetation species works better to reduce PM concentration. The implication of this research is that road greenbelt development should systematically consider surface water control, noise reduction, recreation, aesthetic, and air pollution control, thus to maximize its ecoservices to human being.