In jointed rock masses deformability is associated with the mechanical properties of the intact rock and the rock joints. It is also known that the deformation modulus of jointed rock masses is anisotropic because of the presence of rock joints. This paper presents an analytical approach to the estimation of the deformation modulus of rock masses that contain any number of single joints or joint sets. The data needed for calculation are the elastic parameters of the intact rock, the orientations of the joints and the stiffness of the joints. The deformation modulus of the rock mass calculated in three-dimensional space can be plotted in a hemispherical diagram. From this diagram one can find the value of the deformation modulus in any given direction and even an estimate of the average overall modulus.