Phase locking of microwave oscillations of two nanosecond Gunn-effect X-band oscillators with a 20-W peak power is investigated. The phase is locked by the leading edge of the voltage pulse from the common modulator at each oscillator diode. It is shown that the standard deviation of the phase difference decreases, as the rate of the voltage rise increases until the oscillation starts, and it can be about 0.1 rad, corresponding to 1–2 ps on the time scale.