The rocks used in the present study are three sedimentary rocks, i.e., shales, sandstones and bituminous coals. The rock cores were obtained from two different coalfields: the North Karanpura coalfield in Jharkhand state and the Raniganj coalfield in West Bengal state in India. The main objective of the present study was to determine the values of mi as per the Hoek and Brown criterion. A nonlinear strength criterion, as proposed by Ramamurthy for the triaxial compression case, has also been used in the present study. For this, different physical and mechanical properties tests are determined strictly as per the Indian standard codes. The mean values of “mi” of the intact rocks as per Hoek and Brown criteria for sandstones, shales, and coals were found to be 10.48, 11.72, and 12.71, respectively. However, it was observed that the “mi” values of the shale and the coal varied widely.