The brittleness is a fundamental rock property and effective in drilling and rock excavation. Knowing the relations between the drillability and the brittleness will help the rock engineers. For this reason, to investigate the relations between the drilling rate index (DRI) and the different brittleness values, 32 different rock types were tested in the laboratory. Uniaxial compressive strength and tensile strength were determined for the brittleness calculations. The brittleness of B-3 (the ratio of compressive strength minus tensile strength to compressive strength plus tensile strength), and the brittleness of B-8 (the half of the product of compressive strength and tensile strength) were calculated from the test results. In addition, Sievers' J-miniature drill test and the brittleness test were carried out and DRI values were calculated. Then, B-3 and B-8 were correlated with DRI values. A good correlation between DRI and the brittleness of B-3 could not be found. However, a strong exponential relation between DRI and the brittleness of B-8 was found. Comparing the results with the literature showed that there was an agreement between this study and previous studies. It can be concluded that B-8 can be used for the assessment of rock drillability. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.