Unaccounted for water(U.F.W) represents water that has been produced and is lost before it reaches the customer, through leakage and, other means. U.F.W is an environmental, social, cultural and economic issue. Because loss exist in any water distribution systems, therefore it is necessary to identify the causes, effects and degrees of the problem  and  find the best way for reduction and control of U.F.W. In this research, the causes of UFW and estimation of it as a case study in the city of Sisakht in Kohkiloye and Boier Ahmad province has been studied. Part of Physical (water loss) and non-physical (apparent loss) water have been presented. The approach method in non-physical section includes identification of, illegal meter connection, meter inaccuracy, errors in water reading systems, estimation of water loss from each sources, and it's effects in total U.F.W. finally the best method for   control of water loss in this part is discussed . In physical part study carried out for identification of real loss causes with implementation of district meter area with installation of pressure logger, flow meter in the network and determination using the method of minimum night flow. According to the results, from 1014700 cubic meter of water produced daily, about 483260 cubic meters is UFW (47.6%) which consists in physical with 349195cubic meter (34.4%) and non-physical with 134065 cubic meters (13.2%). With respect to the national scale, which is 31%, the UFW in Sisakht is 16.63% higher.