The effects of three Arbuscular Mycorrhizal Fungi (AMF), Glomus versiforme, G. intraradices and G. etonicatum) and three irrigation intervals (7, 9 and 11 days) on growth of onion (Allium cepa L.) cv. Red Azar Shahr were studied under two soil conditions (sterilized and non-sterilized). The results indicated that, AMF colonization improved plant height, Leaf Area Index (LAI), total biomass, bulb dry mass and diameter, Harvest Index (HI) and chlorophyll content (p < 0.001). Bulbing occurred 10-15 days earlier in mycorrhizal plants. Irrigation interval decreased biomass, LAI, Leaf Area Ratio (LAR), bulb diameter and dry mass and chlorophyll content (b and total) at 11 day irrigation interval. In term of interaction, G. versiforme at 9 day and non-mycorrhizal plants at 11 day produced the greatest and the lowest LAI (8.56 vs. 1.57), respectively. Mycorrhizal onions in contrary to non-mycorrhizal ones produced more LAI and biomass in sterilized soil and inoculation with G. etonicatum and the non-mycorrhizal onions in sterilized soil had the highest and the lowest biomass, respectively.