In this paper, a model is used to harvest energy from the piezoelectric. The harvested energy is in large uncertainties such as uncertainties caused by the material properties, loading conditions and the uncertainty resulting from the tolerances sensitive. Therefore, in order to achieve the proper functioning on the uncertainties, the reliability based design optimization is planned. The results show that if reliability base design optimization method with to the uncertainty caused by tolerances to be done for energy harvesting systems, power output requirement and also ensure the proper functioning significantly increased when compared with the stochastic optimization. In this study, a piezoelectric vibration energy harvester designed to replace the battery supply for actuators and sensors in the unmanned aircraft with respect to predetermined power requirement. In this case, the vibration source is the aircraft engine vibrations.