Abstract: To recognize the structural characteristics of urban green space, this study analyzed the composition and distribution of alien species in built areas in Beijing. We systemically investigated the plant sources using the method of mechanical layout and random sample. We found 173 alien species and approximately 20 invasive species in green-space areas of Beijing. Domestic introduced species constitute a great part of the alien s pec ies; the dominant ex otic plants originate from Central America and Eurasia. Some quantitative disparity of alien species exists for different green-space types, whereas the disparity of invasive species was indistinctive.