This paper presents an impact based piezoelectric vibration energy harvester using freely movable metal spheres as proof mass and an MFC (Macro Fiber Composite) beam as a piezoelectric cantilever. External vibration is transformed into free motion of metal spheres inside the channels and impact between the metal sphere and channel end induces a vibration of the MFC beam to generate electric power. A proof-of-concept device having the form-factor of a conventional wristwatch has been designed and tested to harvest energy from low frequency human-body-induced vibrations. Geometry at both ends of the cavities has been optimized to effectively transmit energy to piezoelectric cantilever and thus achieve a higher output power. For a device having an effective volume of 4.59cm3, maximum peak-to-peak open circuit voltage of 54V and RMS output power of 621μW have been achieved, when the fabricated energy harvester was mounted on a wristwatch and shaken manually.