Input parameters for design of underground openings are not defined properly, such as rock mass parameters and deformation modulus. Available uncertainty in these parameters is intrinsic or misunderstanding of these parameters. To quantify these parameters to design underground openings it must be utilized the statistical methods. In this research, it is used a statistical method to define GSI of rock mass, block volume and joint conditions. By using GSI distribution function obtained from field data and intact strength characteristics, statistical distribution function of rock mass parameters (deformation modulus and shear strength parameters) are defined with the use of Monte Carlo method.