In this study, a new model was presented for computing strength of rock masses based upon in-situ observations of RQD popularly known as rock quality designation. This model links up the rock mass parameters from in-situ investigations with the strength parameters of jointed rocks obtained from laboratory scale experimental observations. Using the constitutive relation, the author derived a pressure and damage sensitive plastic parameter to determine strength of rock masses for varied extents of discontinuity and pressure induced damage. The test results show that plasticity characterized by hardening and softening inclusive of damage invariably depends upon mean pressure and extent of deformations already experienced by rock masses. The present work explores the test data that reveal the dependence of in-situ strength on incremental joint parameters obtained from the joint number, joint orientation, joint roughness, gouge parameters and water pressure. Substituting the relationship between the RQD and modified joint factor with that between modulus ratio and strength ratio, the model shows successfully that using damage inclusive plastic parameter and RQD provides a relationship for estimating the strength of rock masses. One of the main objectives of this work is to illustrate that the present model is sensitive to plasticity and damage together in estimating in-situ strength of rock masses in foundations, underground excavation and tunnels.