Abstract A good understanding of thermal effect on physical properties (especially the permeability) of host rock is essential for a high-level radioactive waste (HLW) deep geological repository. In this study, we carried out comprehensive tests (permeability measurement, p-wave velocity measurement, mercury intrusion test, and nuclear magnetic resonance [NMR]) to study the macroproperties and microstructures of Beishan granite after different temperature treatments. We found the permeability decreased after treatment at 300°C then increased sharply with increasing temperature as temperature exceeded 400°C. Results from the mercury intrusion test and NMR measurement showed that the pore size and pore volume both decreased for granite after being treated at 300°C, which indicated that pores closured around 300°C for Beishan granite. From 400°C upward, the pore size and volume gradually increased with temperature. Based on the relationship of measured T2 data from NMR with Pc curves from mercury intrusion measurement, the synthesized Pc curves was constructed using NMR-measured data of thermally treated Beishan granite. The fractal dimension of pore structure was studied based on pore size distribution from NMR data. Consistent with the evolution of permeability, the fractal dimension decreased for granite after treatment at 300°C, then increased with temperature from 300°C to 700°C. Because the fractal dimension of thermally treated granite is effective in characterizing the variation of pore structures, a semiempirical formula is proposed to predict the permeability using the fractal dimension and porosity.