Abstract: Accurately evaluation of brittleness characteristics of granite under different water content is of great significance to rock mass stability evaluation. The existing rock brittleness evaluation indices are summarized. The indices based on stress-strain curve are analyzed in detail. The brittleness of granite decreases with the increase of water content under uniaxial compression,but experiment results show that the indices which based on the stress-strain curve are difficult to accurately reflect the brittleness characteristics of granite under different water contents. So a new brittleness index B_d is proposed and can fully reflect the whole process of granite deformation and failure. Considering the whole process stress-strain curve and the post peak failure time,the new index B_d uses the peak strain to characterize the pre-peak brittleness characteristics and uses the post peak stress drop rate and the post peak strain growth rate to characterize the post-peak brittleness characteristics. It is proved by experiments that the index B_d can accurately reflect the trend that the granite brittleness decreases with the increase of water content. The new index B_d has superiority over the other brittleness indices. The research results can provide some references and help to enrich and improve the rock brittle characteristics evaluation methods.