An experiment was conducted on stored onion pests in Fayoum governorate to determine the main insect pests, symptoms of damage, population fluctuation to evaluate its effect on yield loss throughout 2008/09 and 2009/10 seasons. The results revealed that Eumerus amoenus Loew, Ephestia cautella (Walker), Carpophilus immaculatus Luc., C. hemipterus F.,and Athrigona orientalis Schin were the main insect pests of onion bulbs in store. The symptoms of damage for E. cautellaand Carpophilus spp were described and population of obtained pests were studied. The roles of these pests in yield loss were considered by weight or either number of infested bulbs (% infestation). The detected pests were arranged descendingly according to the infestation percentage in 2008/09 and 2009/10 seasons as: E. amoenus (10.8& 8.3%), E. cautella (8.8&7.7%), Carpophilus spp. (7.2&6.0%) and A. orientalis (2&1.5%). The observed pests often destroy the infested bulbs completely except E. cautella. The weight loss percent caused by the insect pests ranged 25.78 and 24.10% of the total weight loss.