Knowledge of geomechanical parameters of soil and rock is important in the design and construction of engineering structures. The modulus of deformation can be determined by in situ tests and indirect methods. In estimating this modulus indirectly, empirical relations are a simple and inexpensive method, but due to changes in the type of rock and the nature of the rock mass, the use of empirical relations in other regions of the world will be mostly accompanied by errors. In this paper, an attempt has been made to estimate the modulus of deformation of rock mass (E_m) of the rock mass of southwestern Iran using the parameters of elastic modulus of virgin rock (Ei) and rock mass rating (RMR). For this purpose, the multivariate linear regression method has been used. The database used includes 333 data. To examine the performance of the relationship and evaluate its accuracy, the coefficient R^2 (coefficient of determination) and RMSE (root mean square error) were used. The coefficient R^2 for the data is 0.811 and the value of RMSE is 0.1921.