The potential-free sensor for the measurement of two-dimen-sional transient electric field and the experiment of its calibration are introduced. With the sensor
the experiment of measurement of electric field was designed when the large air gap discharges occurred under switching impulse voltages. The temporal and spatial change of the impulse electric field was measured quantitative-ly when a leader occurred in the gap. The characteristic of leaders was acquired qualitatively by the measurement of the transient electric field. The test results are in agreement with the theory in literature.