Greenbelt along road has three effects, such as traffic safty, improvement of landscape and preservation of environment. In this paper, amelioration effect was considered. In order to investigate absorption effect of plants, many species of plants were fumigated with NO2 in a growth cabinet. In order to explain diffusion effect of greenbelt, field measurement of NOx concentration was made on the roadside where greenbelt is prepared or not. According to both kinds of research result, amelioration of air pollution by greenbelt become clear.