Realization of self powered wireless sensor network has become possible with the recent development in energy harvesting and micromachining techniques. The versatility of vibration energy makes it the most attractive power source for wireless sensors. In several environments, vibration energy is distributed in few harmonics. To effectively scavenge ambient energy, one has to choose the harmonic to which the harvester should be tuned. This paper analytically investigates dependence of the average power of a vibration energy harvester on the frequency of vibration. It is shown that the cubic dependence of the average power on the frequency of vibration holds only when the electromechanical coupling can be increased with the frequency. For a constant electromechanical coupling, the power increases with the fourth power of the driving frequency. When the magnitude of the base acceleration is constant, a high frequency harmonic will give more average power density than a low frequency one. © 2012 IFSA.