We have made observations of a heat signature that is associated with the frictional heat generated at the time of faulting for a large earthquake. Temperature measurements in a borehole that intersects the Chelungpu fault at a depth of about 1100 m, show a small increase near the fault even six years after the earthquake. The temperature signature has a symmetric shape with a width of about 40 m and is centered on the fault that slipped about 5 m during the 1999 Chi-Chi, Taiwan earthquake. The small amplitude of 0.06¿C for the observed temperature anomaly indicates a very low level of friction that generated heat at the time of the earthquake.