We study the important role of temperature rise in the dynamic weakening of fault gouge at seismic slip rates by using host blocks composed of brass, stainless steel, titanium alloy, and gabbro with thermal conductivities (λh) of 123, 15, 5.8, and 3.25 W/m/K, respectively. Our experiments are performed mostly on fault gouge collected from the Longmenshan fault, Sichuan, China, consisting primarily of illite and quartz. High-velocity weakening of gouge becomes more pronounced as λh decreases because the temperature in the gouge increases. Microstructure observations reveal welded slip-zone material and more compact slip surfaces for the gouge deformed with low-λh host blocks, which is probably caused by a sintering process indicative of higher temperatures. These conclusions are supported by temperature calculation performed using the finite-element method. The observed differences in frictional behaviors, deformation microstructures, and calculated temperature demonstrate that temperature rise driven by frictional heating is essential in causing dynamic weakening of gouge at seismic velocities. We show that our data are in good agreement with the flash-heating model, though thermochemical pressurization may also be important. Some of our experiments, where nanoparticles are present but show negligible weakening, demonstrate that the presence of nanoparticles alone is not sufficient to cause dynamic weakening of faults.