In this work, we focus on the experimental evaluation of the bipolar surface potential in a MEMS electret vibration energy harvester. Bipolar charging increases the absolute electrical energy of an electret when the electrostatic force is kept low. Measuring the practical surface potential is necessary for the design and optimization of the harvester. However, it is difficult to estimate the potential after assembly because the typical approach using surface potentiometers is not applicable. In this work, an evaluation method for bipolar charging in conjunction with an external voltage source was developed. To evaluate the proposed method, a typical MEMS electret vibration energy harvester was fabricated. In the experiment, the harvested voltage and displacement at a low acceleration of 0.2 mg (gravitational acceleration, g = 9.81 m/s2) were measured. Since the proposed method is applied at harvesting, the harvested voltage can be changed with an external voltage. The proposed method shows that factors including the relative responses of the harvested voltage can be estimated using the practical potential. In this work, the surface potential in the electret was explicitly considered in the harvesting experiment using a practical bipolar MEMS electret-based vibration energy harvester and an external voltage source.