Abstract: This study deals with rock-bit interaction mechanism in rotary drilling to assess rock drillability for hard rock formations. A serial of drilling tests with different thrusts and rotary speeds were separately conducted on the cylindrical specimens of granite, marble and limestone by a newly developed drilling monitoring equipment. During the test, the rock-bit interaction performance and drilling parameters(e.g., thrust, rotation speed and torque) were recorded by a high-speed camera and the drilling monitoring equipment respectively. The mass and size distribution of rock chips were finally measured to analyze the factors controlling rock fragmentation. The results of this study show that the drill bit presents a helicoidal trajectory in rock because of the simultaneous indentation and cutting process, resulting in rock crushing, shearing, and tensile cracking respectively. Some of the large size chips burst due to large absorbed cutting energy, but most of chips accumulate in the cutting path of the drill bit and increase the cutting friction energy. Additionally, the applied thrust is the essential factor in rock fragmentation, the rock compressive strength and tensile strength contribute significantly to the mass of rock chips and size of rock chips respectively during rock fragmentation. There is also a positive linear correlation between torque and thrust. The magnitude of this linear slope is affected by the cutter movement condition, but not by the rotation speed. A higher linear slope specifically means bigger rock abrasivity and rock intrinsic energy. A new rock drillability theoretical model was finally proposed on the basis of those rock-bit interaction results, which is not only related to rock intrinsic specific energy and friction coefficient but also independent of drilling parameters, such as thrusts and rotation speeds. The validity of this theoretical model in practical engineering applications was also verified by drilling data available in the literature.