In order to improve effectiveness of plant biodiversity conservation and provide scientific bass for nature reserve planning in Beijing, we conducted priority area assessment for plant diversity in Beijing mountains, Form 2001 to 2005, data on 25 rare and endangered species, communities with higher species abundance, larger distributed areas and more important ecological function were analyzed based on 1168 field surrey spots in 13 nature reserves, forest park or scenic areas in Beijing. Results suggest that: 184474.64hm2 should be set as conservation priority areas, which account for 1125% of Beijing territory. Further analyses indicate that among these priority areas, we identified 24850.50hm2 as first grade areas, 78606.60hm2 as second grade areas, and 81017.54hm2 as third grade areas. Most of these priority areas located at far suburb Beijing, e.g., Huairou district, Yanqing County, Fangshan district, Miyun County. Based on our results, a revised nature reserve development plan has been proposed.