High-sensitivity and miniaturized measurement of electric fields is important for applications in power grid such as lightning waveform capturing and electrical equipment monitoring. In addition, the demand of wide-area information measurement in smart grid also necessitates the development of miniaturized and noncontact sensors. So far, there have already been a variety of electric-field sensing devices. However, these approaches meet the challenges of large size and low sensitivity. To solve the problem, this paper proposed a new electric-field sensor based on piezoelectric effect. The piezoelectric effect enables the conversion of electric fields into piezoelectric material deformations and then the air-gap capacitances in the device. The results indicate that the electric-field micro sensor can reach a high sensitivity of 30 V/m. The side length of the sensor chip can be as small as a few millimeters. Owing to the feasibility in mass production of the proposed sensor, it can potentially be extended to the industrial applications in power grid.