Prediction of the drillability of jointed rock mass in excavation with full-section tunnel boring machines is one of the important issues in the technical and economic evaluation of a tunneling project. Considering the effect of various parameters on the drillability of rock mass, various models have been proposed to predict this issue. The aim of this research is to present a new model based on the development of the rock fracture index for jointed rock mass in order to predict its drillability. For this purpose, initially, a database of geomechanical parameters, geometric characteristics of joints (spacing and angle) and operational characteristics of the boring machine of two projects, the Queens Water Transfer Tunnel in the United States and the Long Zagros Tunnel (Section 2) in Iran, was formed. Then, the total crushing factor (presented by Broland in 1979) was calculated and, using SPSS software and nonlinear regression methods, the fracture index obtained in the laboratory for jointed rock mass was developed. Furthermore, the drillability index was developed based on the fracture index of the jointed rock mass, and the correlation coefficient of this index is 0.8. The new model was evaluated in the northern section of the Kerman water transfer tunnel, and the results indicate that there is a good match between the predicted drillability and the actual drillability.