A new type of partial-dielectric-loaded helical groove slow-wave structure (SWS) for millimetre wave travellingwave tube (TWT) is presented in this paper. The radio-frequency characteristics including the dispersion properties,the longitudinal electric field distribution and the beam-wave coupling impedance of this structure are analysed. Theresults show that the dispersion of the helical groove circuit is weakened, the phase velocity is reduced and the positionof the maximum Ez is moved from the mouth to the inside of the groove after partially filling the dielectric materialsin the helical groove SWS. Therefore, the dielectric-loaded helical groove SWS is suitable for a multi-beam TWT withbroad band and high gain.