Ghordanloo dam on the Atrak river, is located about 36 km of northwest of Bojnord city, in the northeast of Iran. This dam currently is at the final stage of the study phase and has been designed as an earth dam with clay core. In this study, engineering geological properties of the rock mass at the dam were investigated in order to evaluate engineering geology properties of the rock masses. The lithology of the rock masses consist of Tirgan formation limestone with lower Cretaceous age that outcrops on both abutments. The thickness of alluvial deposit at the dam axis is 61 meters and it consists of river bend of fine grained and coarse grained materials. The rock mass investigation of the site is performed in both filed geology and laboratory studies. Field study includes the analysis of discontinuity system, boreholes exploration, performing permeability test and sample extraction for laboratory studies. The laboratory study consists of performing uniaxial compression test, Young’s modulus, unit weight, water content and porosity. Based on the obtained results, the permeability of the rock mass is high in the both abutments. According to the results of Lugeon test and borehole log, the relation between Lugeon and RQD were performed. The obtained results showed that there is no meaningful relationship between Lugeon unit and the RQD parameter. Finally, the rock mass of the dam site was classified in accordance with RMR classification

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