The deformation modulus of a rock mass is an important input parameter in any analysis of rock mass behavior for geotechnical projects such as dams. The determination of this parameter by in situ tests are time consuming and expensive. Therefore, different researchers have proposed empirical relationships for estimating the value of rock mass deformation modulus. This work has been done by using rock mass classification systems and data collections from in situ tests in different places. Based on data collected from a large number of dilatometery tests conducted in seven dam sites located at southwest Iran, a new nonlinear multi-variable empirical equation (logistic function) was developed using the GSI index and the rock elasticity modulus (E_i) to estimate the rock mass modulus of deformation (E_d).