The flame boundary is an important geometrical characteristic for the evaluation of flame properties such as heat release rate and radiation. Reliable and accurate measurement of the flame boundary is desirable for the prediction of flame structure and the optimization of combustion systems. Such measurement will inform the designers and operators of the combustion systems. This article presents for the first time a study of using an electrostatic sensor array for flame boundary measurement. The electrostatic sensor is placed in the vicinity of the flame to sense its movement through charge transfer. The principle, design, implementation, and assessment of a measurement system based on this methodology are introduced. Comparative experimental investigations with a digital camera conducted on a laboratory-scale combustion test rig show that the electrostatic sensor can respond to the variation of the distance between the electrode and the flame boundary. Reconstruction of the flame boundary is achieved using a set of distance measurements obtained from a sensor array. For diffusion flames over the range of fuel flow rate 0.60-0.80 L/min and premixed flames over the range of equivalence ratio 1.27-3.81, experimental results show that the measurement system is capable of providing a reliable measurement of the flame boundary. The correlation coefficients under all test conditions are mostly larger than 0.96, the mean relative errors within 7.4%, and the relative root-mean-square errors within 0.09. More accurate flame boundary measurements are achieved for diffusion flames. In addition, the overall polarity of charges in a flame can be determined from the polarity of the sensor signal.