Prediction of utilization based on rock mass properties is critical for any mechanized tunneling project, since the utilization have an influence on duration and costs of project. Using some case studies including Queens, Milyang, Manapouri, Karaj, Varzo, Maen, Pieve mechanized tunneling project around the world, the attempt is made to estimate utilization via Rock Mass Rating (RMR) System herein. It is concluded that the machine utilization has some reasonable correlations with RMR; however, this relationship is depend on not only rock mass properties but also machine specifications. In other words, the utilization may change with both rock mass conditions and utilized machine type. Due to that rock properties and other related downtimes should be examined together to obtain the reliable equations for predicting the utilization. Further, developed relationships may be used for similar rock type to estimate utilization from RMR.