demand for different usages. So dam building has been increased during the last 30 years in Iran. Nowadays, awareness about the impacts of water resource development projects on river's natural flow regimes and river ecosystems has been increased. Meanwhile, a component of environment is known as a legitimate user of water and estimation of environmental water requirement is assigned as one of the most essential parts of the environmental impact assessment and water resource planning reports of dams in Iran. Hence, in 6^th commission of water and environment (2005) in Iran this issue is determined as an enactment and terms of reference for assessing EFR, but as a result of lack of acquaintance regarding EFR assessment methods and incongruity with Iran’s environmental and managerial condition, it has not been implemented completely yet. As at the present time, some simple hydrological methodologies generally have been used for EWRA. Therefore, the most important aim of this study is to identify firstly the features, advantages and disadvantages of environmental flow assessment methodologies (EFAM) which are used in the world; and secondly finding about the fact that whether these methods are adaptable to the conditions of EFA in downstream of dams in Iran. Accordingly, after reconnaissance of different EFA methodologies, the 26 methods among the most applicable and important ones were selected. Then, each of these methods was evaluated in accordance with the adoptability to 45 comparative criteria. Finally the suitability and priority of these methods for application in Iran was designated by using TOPSIS technique. The result of comparison demonstrates that Holistic approach especially DRIFT method has the most conformity with the situation of environmental water requirement assessment of rivers in Iran. Thus, this method has been selected as the proposed approach