Abstract: Based on the test data from a drilling process monitoring system(DPM) instrumented with a hydraulic rotary drill rig in weathered granite formation,kinetic energy,energy from effective thrust force(ETF),energy for viscous frictional resistance(VFR) and total penetrating energy were analyzed for the diamond drilling system.It was shown that the kinetic energy of the drill string system was dominant and energy from EFT as well as VFR was very low in proportion of drilling energy.Besides,a negative correlation between the kinetic energy,the total penetrating energy and the weathered extent of granite existed and a positive correlation between the energy from ETF and the weathered extent was presented,indicating that there was a good response between the drilling energy and the weathered extent of rock.Finally,in a view of specific energy,the relationship between the specific energy of penetrating(SEP) and the weathered extent of rock was analyzed.It was shown that the SEP in diamond rotary drilling was apparently lower than the specific energy of fracture(SEF) in percussive drilling in very strongly weathered and strongly weathered granite formation.However,the SEP in diamond rotary drilling was higher than the SEF in percussive drilling in lightly weathered hard as well as in extremely hard rock.Then a theoretical foundation for classification of rock in the diamond rotary drilling and some new approaches for real-time-rock-classification and real-time-strata-decision were provided.