Abstract: Lost circulation is a common downhole problem of drilling in geothermal and high-temperature, high-pressure (HTHP) formations. Lost circulation material (LCM) is a regular preventive and remedial measure for lost circulation. However, conventional LCMs seem ineffective in high-temperature formations. This may be due to the changes in the mechanical properties of LCMs and their sealing performance under high-temperature conditions. To understand how high temperature affects the fracture sealing performance of LCMs, we developed a coupled computational fluid dynamics-discrete element method (CFD-DEM) model to simulate the behavior of granular LCMs in fractures. We summarized the literature on the effects of high temperature on the mechanical properties of LCMs and the rheological properties of drilling fluid. We conducted sensitivity analyses to investigate how changing LCM slurry properties affected the fracture sealing efficiency at increasing temperatures. The results show that high temperature reduces the size, strength, and friction coefficient of LCMs as well as the drilling fluid viscosity. Smaller, softer, and less frictional LCM particles have lower bridging probability and slower bridging initiation. Smaller particles tend to form dual-particle bridges rather than single-particle bridges. These result in a deeper, tighter, but unstable sealing zone. Reduced drilling fluid viscosity leads to faster and shallower sealing zones.