Onions (Allium cepa L.) are biennial herbaceous plant of the family Alliaceous that are typically grown as annuals. Globally, onions were produced at nearly 35 millions tones per annuals. In Ethiopia, it is widely grown in the rift valley and lakes regions of the country. The objective of this resarch is to review the effects of different level of phosphorus on growth and yield of onion. Phosphorus fertilizer is one the most complex production in many tropical soils, owing to low native content and phosphorus immobilization with the soil. In onion, phosphorus deficiencies reduced root and leaf growth, bulb size and yield and also delay maturation. In the soil the amount of phosphorus is moderately low. Onion growth and yield can be enhanced by applying phosphorus. The use of optimum phosphorus fertilizer is one of the prominent to increase production. Still optimum rate of phosphorus fertilizer is not well identified. To optimize phosphorus availability through phosphorus addition, soil factors must be considered and research should be focused on the level of phosphorus on growth and yield of onion.