An analysis has been made for the high‐field domain dynamics in a planar Gunn diode, based on stroboscopic observations by a scanning electron microscope. The 1‐GHz pulsed electron beam was synchronized with the microwave‐triggered oscillating diode. The movement of the domain could be clearly observed. The domain dynamics is discussed in detail, taking the time fluctuation in microwave triggering into account. The experimental results are also supported by a theoretical analysis.