The effects of the presence of calcite grains on the mechanical behaviour of a coal are investigated in this study. Coal specimens with and without calcite grains were obtained from the same coal seam and then subjected to confined compression with varying confining pressures. Mineral composition data of the coal containing calcite grains were obtained from petrographic analysis and subsequently used for constructing UDEC-GBM model. The model was calibrated to experimental results and then used to further the understanding of the influence of calcite grains on the mechanical behaviour of the coal. It is found that the presence of calcite increases the Young's modulus, reduces the peak strength and the brittleness of the coal. The numerical results suggest that the presence of calcite grains significantly increase the heterogeneities of the coal, leading to differential and extensional straining which in turn causes internal tensile stresses and consequently tensile cracking. The conformity of the numerical results with the experimental observations suggests the potential use of the UDEC-GBM approach model for better understanding the mechanics of brittle rock failure at micro-structure scale. (C) 2016 Elsevier B.V. All rights reserved.