This study was carried out during 2008/2009 and 2009/2010 seasons to evaluate the phytotoxicity and competitive effect of some weed species (Ammi majus L.), ( Rumex dentatus, L.), (Chenopodium album L.), (Convolvulus arvensis, L.) and (Melilotus indicus, Ten.) on seed germination, seedling vigour, growth characters and yield of onion. Moreover, to evaluate the efficiency of different control methods (chemical and hand hoeing) against onion weeds. The results showed that, (seeds + roots) exudates of the different tested weeds caused a significant decrease in seeds germination and seedling vigour characters of onion, i.e., seedling length, shoot length, root length and seedling fresh weight. On the other hand  results of  the competitive effect of weed species on growth characters and yield of onion, showed that,  the growing tested weeds with onion decreased significantly onion growth characters, i.e plant height, number of leaves/plant, bulb and neck diameter and dry weight/plant compared to control treatment (onion grown alone). The competitive ability of (Ammi majus) was more  pronounced than the other tested weeds on the growth of onion as well as the average bulb weight at harvest. Also,  results indicated that all herbicidal treatments showed significant efficacy against total weed during both seasons. The most effective treatment against total weeds was hand hoeing twice, followed by pendimethalin, oxadiargyl and Metribuzin + fluazifop-P-butyl, respectively. All tested herbicidal treatments significantly increased onion growth characters during the growing stage and at harvest during both seasons. Bulb onion yield/fed significantly increased to different extents as result of using the tested herbicidal treatments in both seasons as compared to control. Generally, it is concluded that the interference (phytotoxicity and competition) of weeds with onion caused an evident harm in the seed germination, growth and yield of onion especially (Ammi majus) and (Melilotus indicus). So, weed elimination in transplanted onion by these potent herbicides can be recommended for weed control in transplanted onion.