Abstract: At the micro-scale, the different minerals, pores, and microcracks in granite create heterogeneity, thereby impacting the formation and progression of microfractures. This study examined the generation and development of thermal cracks in a Westerly granite sample, using high-resolution, in-situ heating synchrotron CT observations from room temperature up to 395°C. The discrete element method was subsequently employed to create models based on the CT images, including different minerals and pre-existing pores and cracks, to simulate the development of thermal cracks. The CT images revealed small pores within and between the granite mineral grains, as well as numerous cracks between them. Newly formed thermal cracks were detected at approximately 100°C with larger cracks appearing at temperatures > 200°C, eventually resulting in a network of cracks. Numerical simulations using the discrete element method demonstrated that thermal cracks primarily develop at the interfaces between the different minerals. The temperature and distribution of simulated cracking correlate well with the CT observations. A comparison with model simulations without pre-existing pores showed that these pores resulted in lower thermal stress within the mineral grains and a lower initial temperature at which thermal cracks formed. This indicates that granite containing pre-existing pores/cracks would be damaged at temperatures of 200°C or lower. These findings enhance our understanding of the development of thermal damage in different granites and offer insights for predicting thermal damage in rocks.