Without water, life on Earth is meaningless, Then, this century would be named the century of struggle for water ownership. Due to rapid population growth and increasing water needs, including drinking, industry and agriculture, Needs for robust management due to optimist use of water resources is undeniable. Nowadays, basic subject of different countries in the world development is water. in the current situation, water. is considered as basic issue in commercial development and social welfare. In all countries, dams with different aims, such as, water supply, power generation, irrigation, flood reduction and etc. have been made about 60% of the world's great rivers are cut by dams and this have affected much on the regional environment and inhabitants those are relevant to this vital matter. This study, is assessed the environmental impacts of Hanna Dam in Semirom, Iran, such as economic, social and ecological effects, from the attitudes of farmers using the dam irrigation network. The beneficiaries of dam irrigation network are 4715 households that by using the Cochran formula, 200 samples are obtained and filled using the data from questionnaires and interviews. the validity of the research tools from experts confirmation and its perpetuity are calculated using Cronbach alpha coefficient assigned to the different parts of economic 84%, social 71% and ecological 75%. The effects of the project have been evaluated by Bayesian networks. In this study, to identifying and evaluating the effects of dam, the all ecological, economic and social impacts of this project are assessed and the cause and effect relationships between the impacts are determined. to get the Bayesian networks Netica software is used Bayesian networks, and evaluating and the effective causes and their impact on the region, are shown by this model. The single-sample t-test results shows relatively satisfaction i the economic part including the whole water allocated to agriculture, area under cultivation increment, improvement of farmers’ incomes, increment the value of the property and fields. In social matter The majority of the beneficiaries are dissatisfaction with the state management of water resources and its distribution , whereas the ecological impact, discharge of the aqueducts and fountains, lush and beautiful landscape has improved. The correlatio (or name of the test name) test results showed a high straight correlation between trust in the authorities and farmers satisfaction of dam building. Moreover, regression analysis results showed that with prediction strength probability about 42% , the effective reasons on farmers' satisfaction of water supply projects in the region could be related to increasing discharge of the aqueducts and fountai (0.255), crops variation, (0.208), reduction water pollution index of (0.206), And fair allocation of water between farmers,(0.176), in order of importance and content Beta respectively. Effects in three categories: high, medium and low are classified, the results show that most impacts in this region are in the high class, with a value of 51.6% euantully values for medium and low class are 40.4% and 8.05% vespectively. The results of the sensitivity analysis for the region final node, represents that economic effects, social impact and ecological effects, respectively have highly impact on building and benifitting of the hanna dam.