The study of micropower supply has become a key issue in the development of wireless sensor networks using microelectromechanical system technology. Vibration-powered energy harvesting system is considered as a good source for generating micropower. A general layout of a vibration energy harvesting system based on a sandwiched (PZT-Si-PZT) rectangle cantilever with a nickel proof mass was proposed. For the optimization of the system, formula of the voltage and power output based upon an equivalent circuit of piezoelectric layer and ac-dc converter was deduced. For the vibration harvesting structure, lower resonance frequencies are usually preferred. In order to find the relationship of the cantilever length and proof mass on the resonance frequencies of the energy harvesting structure, computations using Microsoft Excel software were conducted. The results show that it would be advisable not to increase the cantilever length only in order to get lower resonance frequency. However, the resonance frequency of the structure can be reduced dramatically by the accession of a proof mass with reasonable weight, so that the whole structure can be optimized to fit its application in natural environment. This study provides some basic considerations for the design of piezoelectric-based vibration energy harvesting system for converting ambient vibrational energy into electricity.