Relative abundance and niche of weed communities in the rape field of Linzhi, Tibet were investigated. Seven families and 11 species were observed in the rape field and Leguminosae, Poaceae and Chenopodiaceae were the three major family. Cirsium lanatum, Plantago asiatica, Avena fatua, Medicago lupulina, Poa annua and Vicia angustifolia were the predominant species. In addition, we identified the variation characteristics of dominant weed species. MidApril was the initial period and early May and late June were the two peaking periods of weed growing. M. lupulina had the widest ecological niche and therefore serious harm to rape. P. asiatica and C. lanatum had certaindegree harm to rape. P. annua and P. asiatica had the largest overlap, which were larger than the horizontal and vertical ecological niches of A. fatua, and had similar utilization of resource. Therefore, we can use relative abundance and niche to identify the dominant weed species and predict their harm to rape.