Deformation modulus of rock mass which is composed of plastic and elastic deformations, are defined as the ratio of stress to corresponding deformation during the loading of rock mass. This parameter is of fundamental importance for a safe and economical design of rock structures, i.e. large underground opening like underground powerhouses. Among the rock mass properties, the deformation modulus has vital importance for the design of rock engineering projects, because it is the best representative parameter for the pre-failure mechanical behavior of the rock material and of rock mass. There are direct methods to determine the rock mass modulus, i.e. in situ tests, and also empirical relations for indirect estimation of the deformation modulus. In large geotechnical projects, determination of this parameter is carried out by in situ tests, which can cover a large volume of rock masses subjected to load. The plate loading test is a mostly familiar in situ test to measure the deformation modulus of rock mass. The determination of this parameter by in situ tests requires considerable costs and involves dif?cult operational processes. On the other hand, there are several emprical relations to estimate deformation modulus, but none of them alone is recommended. In this study, Neuro-Fuzzy modeling is performed in order to evaluate the deformation modulus of rock mass in Beheshtabad dam site. At first, the results of the plate loading tests have been analyzed using different methods and the most appropriate one is applied in the output of the model. Then, all the effective parameters in the modulus of the rock mass including field and laboratory tests evaluated by linear regression. To evaluate the existing empirical equations, RMR ltr"
