<p>Geothermal energy extraction from hot dry rock is often hindered by its inherently low permeability and the technical challenges involved in creating and sustaining fracture networks. Liquid nitrogen fracturing has emerged as a promising technique to overcome these challenges; however, the evolution of rock brittleness during cyclic thermal treatment remains poorly understood. This study introduces a new brittleness index to quantify the progressive changes in rock brittleness during the cyclic treatment. The brittleness index is derived from a linear correlation between fracture surface area and energy release rate, identified through a series of three-point bending tests on notched semi-circular samples. Compared to existing brittleness indices, the brittleness index exhibits a sharp increase during the initial cycles, followed by a gradual increase as the number of cycles continues to rise. The brittleness evolution aligns well with physical observations, such as mass loss, P-wave velocity reduction, and fracture surface evolution. Acoustic emission monitoring further enables the differentiation of the brittleness index into tensile-dominated and shear-dominated brittleness indices, providing valuable insights into the competition between tensile and shear cracking during the cyclic treatment. The applicability of the brittleness index to geothermal energy extraction is demonstrated by considering hot dry rock subjected to liquid nitrogen fracturing at varying depths. Our analysis reveals that liquid nitrogen fracturing substantially enhances rock brittleness, particularly in deeper rock with higher temperature. These findings suggest that the brittleness index can serve as a valuable tool for optimizing geothermal energy extraction by improving fracture network development while addressing key challenges associated with geothermal reservoirs.</p>