Rough Sets theory is a mathematical approach for analysis of a vague description of objects presented by a well-known mathematician, Pawlak (1982, 1991). This paper explores the use of Rough Sets theory in site location investigation of buried concrete water reservoirs. Making an appropriate decision in site location can always avoid unnecessary expensive costs which is very important in construction projects such as water reservoirs. The proposed site location investigation approach is illustrated using a case study data of a semi-buried concrete reservoir with the capacity of 15000m3 which is under construction in the North of Iran (Guilan Province, Maklavan). In this approach, the decision rules are derived from conditional attributes in Rough Sets analysis, in accounting for data vagueness and uncertainty in potentially reducing data collection. The results of study indicate that using this method can reduce unnecessary costs in water reservoirs construction.