Thermal cracking behavior of granite at high temperature and high pressure (HTHP) is the key to the performance of Hot Dry Rock (HDR) geothermal energy extraction system. In this study, permeability tests accompanying acoustic emission (AE) tests in granites are first conducted under HTHP by 600 degrees C 20MN servo control rock triaxial testing machine. The test results show that granites, nearly impermeable rocks, can show a striking increase of permeability by heating from the critical temperature. The growth curve of granite permeability shows two phases because of the multi-period of thermal-cracking in the heating process from room temperature to 500 degrees C. The coupled effect of temperature and pressure shows that critical temperature of permeability change decreases with increasing confining pressure. Then, a detailed characterization of the sample microstructure is presented using Micro-CT method. It is discovered that thermal cracking mainly occurs at grain boundaries in forms of inter-granular micro cracks along apparent weaknesses, and develops with increasing temperature. Meanwhile intra-granular cracks are observed when heating to 500 degrees C, indicating that thermal cracking in granite under HTHP is induced by both intra-granular and inter-granular thermal stress. At last, experimental stimulation and application for geothermal mining of HDR are discussed. (C) 2017 Elsevier Ltd. All rights reserved.