We report, for the first time, the observation of a shear-induced decomposition of boron carbide into B50C2 and nanocrystalline graphite at pressures from 1.0 to −3.5 GPa. It is proved that shear under modest compression provides finer controllability and more effective initiation capability than either compression alone or compression under high temperature. Most importantly, shear, as a driving force, is proved capable of overriding materials’ energy surfaces and thus realizing new chemical reactions as well as structural transformations that have not been discovered. Consequently, shear is of great significance shedding light on new technologies for both material synthesis and advances in material sciences.