This study was conducted in 2019 in order to determine the weed species, their densities and frequency of occurrences in the onion production areas of Ankara and Çorum Provinces. The surveys were carried out in 78 fields, 55 in Ankara and 23 in Çorum. As a result of the surveys, 75 weed species, two of which are parasite, belonging to 28 families and 64 genera, were determined in the onion fields of Ankara Province. The families with the most species were listed as Asteraceae (11), Poaceae (9), Fabaceae (6), Chenopodiaceae (5) and Apiaceae (5). Other families were determined between 1-3 species. Of these weeds, Convolvulus arvensis L. (1.2 plants/m2), Xantium strumarium L. (0.50 plants/m2 ), Amaranthus retroflexus L. (0.40 plants/m2), Cuscuta sp. (0.37 parazited onion/m2), A. blitoides S. Watson (0.36 plants/m2) were the 5 most intense species. According to the frequency of occurrence, the first five species are determined as; C. arvensis L. (90.90%), X. strumarium L. (65.45%), A. retroflexus L. (54.54), Chenopodium album L. (52.72%), A. albus L. (47.27%). In the onion cultivation areas of Çorum Province, 61 weed species one of which is parasite, belonging to 24 families and 51 genera were determined. The families with the most species were listed as Asteraceae (9), Poaceae,(8) Apiacea (5), Polygonaceae (4), Fabaceae (4); as for other families, between 1-3 species were determined. Considering the densities per m2 of these weeds, the 5 most common weed species are determined as C. arvensis L. (0.80 plants/m2), Anethum graveolen L. (0.56 plants/m2), A. retroflexus L. (0.41 plants/m2), Chrozophora tinctoria (L.) Raf. (0.32 plants/m2), X. spinosum L. (0.31 plants/m2), the first five species in terms of frequency of incidence are; C. arvensis L. (86.95), A. refroflexus L. (56.52%), X. strumarium L. (56.52%), Cirsium arvense (L.) Scop. (52.17%), C. album L.(52.17%).