Abstract: Based on the Lajtai’s theory and Jennings’ theory,the mechanical behavior of brittle rock mass containing coplanar close discontinuous joints under direct shear condition was analyzed.Three failure models were proposed,and the mechanism of the models was also analyzed.Through modifying the Lajtai’s theory,the failure strength criteria of such rock mass were proposed.The shear strength of jointed rock mass under the combined action of shear and tension was determined.The proposed mechanism could explain the phenomenon in direct shear test,and the calculated results coincided well with experimental results.