The coalfield fire is determined by fractures of coal and rock that provide tunnel for gases and heat exchange. To study fracture propagation at high temperatures, high-resolution X-ray computed tomography (CT) was used to scan anthracite and mudstone samples collected from the Qinshui coalfield, Shanxi Province, northern China. The samples were scanned at 100 °C intervals as they were subjected to temperatures of up to 500 °C. Three-dimensional images were reconstructed by the CT software to analyze changes in the fractures and pores in the samples. The experimental results show that fracturing of anthracite began at 200 °C. The generation rate of fractures in the coal samples increases slowly below 300 °C, but above 300 °C there is a sharp increase in fracture development. This indicates that the thermal fracturing temperature threshold for anthracite is 300 °C. During the experiment, it was found that preexisting fractures, voids, and regenerative fractures formed around the hard portions of anthracite particles or along the weak boundaries between particles. Some regenerative fractures developed along the fabric of the relatively crystalline particles within the particle and terminate at the edge of the particle or where the fracture encounters a harder portion of coal. Some fractures even expanded enough to be transformed into voids as temperatures rose. In the mudstone, the porosity changed suddenly at 300 °C. This indicated that there was a void generated at 200 °C, but the void expanded when the temperature was increased. However, changes in the void were not obvious from 200 to 300 °C.