The frictional heat generated during an earthquake is thought to be a major portion of the total energy release. However, there have been no direct measurements of the heat generated by a large earthquake. We present an estimate of the frictional heat produced by the 1999 Chi-Chi, Taiwan earthquake (Mw 7.6), from temperature anomaly that is measured in a borehole penetrating a large slip region (∼10 m) of the fault. A local increase in the temperature profile across the fault is interpreted to be the residual heat generated during the earthquake. Analyses of this temperature anomaly lead to a low estimate of the dynamic shear stress, 0.5 to 0.9 MPa, and the heat produced in the large slip region of the fault, 0.68 to 1.32 × 1017 J. The total frictional energy is estimated as 2.4 to 6.1 × 1017J, indicating that seismic efficiency is 1 to 3%.