Abstract: Based on experimental data from a drilling process monitoring system instrumented with a hydraulic rotary drill rig in fill-weathered granite formation, the diamond penetrating energy was analyzed. The result shows that the distribution of the penetrating energy in fill-weathered granite formation agrees with that in common weathered granite formation. A negative correlation between the viscous energy, kinetic energy, total penetrating energy and the weathered degree of granite existed, and a positive correlation between the thrust force energy and the weathered degree was presented, which indicates that there is a well response between the penetrating energy and the weathered degree of rock. However, the kinetic energy, thrust force energy and viscous energy are limited in identification of formation because of different effects in various drilling modes. The specific energy of diamond drilling (SEDD) increased with the weathered degree of rock and the values of SEDD can be classified into corresponding range according to the weathered degree of rock. It is shown that the SEDD in rotary drilling is apparently less than the specific energy of percussive drilling (SEPD) in both fill soil and very strongly weathered granite formation. Reversely, the SEDD is much more than the SEPD in lightly weathered hard rock.