Abstract: The phase-locking process is studied for high-power gyrotron oscillator driven by an external signal.The phase-locking nonlinear differential equations are derived, the condition of phase-locking operation and these features of phase-locking process are analysed.The phase-locking signal can be introduced after gyrotron oscillatinginto saturation or before it. Two different ways of inputing signal make markable influence on the process,the phenomenon is discussed. In this paper, the numerical calculations and analysis are given for gyrotron T₁₃ mode.