Fault rocks share certain characteristics of melt-origin pseudotachylyte due to elevated temperature caused by coseismic frictional heating. However, there is no broadly accepted quantitative evidence to identify signatures of coseismic frictional heating in fault rocks. We report systematic magnetic studies on a brown ultracataclasite layer in the rocks from Longmen Shan thrust belt, at the eastern margin of the Tibetan Plateau in Sichuan Province, China. The brown ultracataclasite has: (1) the highest magnetic susceptibility, (2) significant characteristics of magnetite neoformation and (3) similar demagnetization behavior of natural remanent magnetization and anhysteretic remanent magnetization. The principal mechanism responsible for the high magnetic susceptibility of the brown ultracataclasite is most likely caused by the production of new magnetites from iron-bearing paramagnetic minerals. These new magnetites can be formed by frictional heating on slip planes along a seismic fault. The study shows that magnetic analysis can help to recognize frictional heating events in specific fault rocks.