More than 20 sheep breeds having different body size utilize various rangelands in different climate regions which the conversion coefficient of each breed compared to animal unit must be recognized. For this purpose, two herds were selected from each breed sheep grazing at the central region during 2001-2007. Among selected herds, 30 animals randomly weighted at various age and sex groups and at various grazing seasons. Mean weight of three and four years old sheep was considered as mature weight of each breed. Conversion coefficient of each breed to animal unit (animal unit equivalent) calculated from the ratio of metabolic weight ((live weight)0.75 to animal unit metabolic weight. According to the results, Naeeini breed classified as light body breeds, Zandi, Kermani and Varamini as medium body and Shaal as heavy body breeds. Equivalent animal unit for each breed to animal unit were 0.7, 0.86, 0.9, 0.97, 1.06 and 1.20. based on proposed conversion coefficient and using MAFF proposed equation, daily requirement of each breed at maintenance condition and at grazing condition were as 4.76, 5.85, 6.12, 6.60, 7.21 and 8.16 MJ per day. Results represent that considering to same forage amount as daily requirement of grazing sheep breed in rangelands in order to calculating grazing capacity, without considering their body size is not valid.