Brittleness is one of the important properties of rock, which is of great significance in rock engineering. The evaluation of brittleness index is a useful way for the estimation of rock brittle. However, there is still much uncertainty about the relation between rock brittleness and rock failures such as hydraulic fracturing, rock bursts in coal mines and tunnels. This fact can be attributed to the lack of the universally accepted brittleness concept and brittleness index. In this paper, the relationship of the rock brittleness and mechanical properties such as the post-peak stress drop coefficient in the stress–strain curve as well as the influencing factors to brittle failure are reviewed. Furthermore, the existing estimation methods for rock brittleness index are summarised and their applicability is briefly discussed. Moreover, the representative applications of rock brittleness index on the hydraulic fracturing, rock burst predictions in coal mines and tunnels were reviewed. Finally, the primary problems in the estimating rock brittleness are pointed out, which are expected to provide some guidance for relevant applications in rock mechanics and engineering.