The present investigations were carried out at Shandaweel Agricultural Research Station, Sohag Governorate, Egypt during 2015/2016 and 2016/2017 seasons to survey onion pollinators, study the population dynamic of the dominant pollinators during the flowering season and their diurnal activity. Also, to make a comparison between seed yield of caged and uncaged (open-pollinated) plants. Data revealed that onion flowers were visited by 21 species of insects belong to 14 families and 7 orders. Of all species 16 species were identified and 5 were not identified 2 species belong to Sarcophaga genus and 3 belong to Syrphus genus. Order Diptera was most abundant 30% followed by Hymenoptera 21%, Lepidoptera, Coleoptera 14%, and 7% for Hemiptera, Neuroptera and Thysanoptera. Of all the insect visitors, Apis mellifera L., Musca domestica (Macq.), Lucilia illustris Meigen and Syrphus spp. were the most abundant visitors. The pollinators were detected during the last week of March till the last week of April. The populations of A. mellifera and M. domestica gave the highest and significant values followed by L. illustris and Syrphus spp. The activity of these pollinators increased gradually at morning and reaches its peak at 12:00 h., with averages of 4.58, 4.39, 4.00 and 2.21 pollinators/25 plants /min. / plant, respectively in the first, and 3.52, 4.18, 3.09 and 2.33 pollinators/25 plants /min. / plant, respectively in the second season. Also, the yield of seeds from plants covered and isolated from insects decreased significantly with an averages of (45.40 and 47.60 seeds/ 20 umbels),whereas open-pollinated plants produced (711.60 and 721.10 seeds/ 20 umbels) at 2015/2016 and 2016/2017 seasons respectively.