Abstract: The small-signal theory of the gyro-TWT, a pinch-point analysis based on the theory of Briggs and Bers combining the cold analysis of the dielectric-loaded waveguide are employed to analysis the threshold current of the operating mode and critical oscillation lengths of the spurious mode under different load condition. The analysis concludes that the improvement of the dielectric loss, through altering the load condition, can enhance the threshold current and critical oscillation lengths. The cold analysis of the dielectric-loaded waveguide and a smooth waveguide dispersion relation is used to analyse the small signal gain of the dielectric-loaded gyro-TWT. Through numerical calculation, the analysis gives the gain-frequency response of a dielectric-loaded gyro-TWT.