Abstract: Base on the Foulds′ theory about the propagation of an electromagnetic wave through a helical waveguide, the effect of the higher-order modes in the groove is taken into account. The dispersion diagram obtained from calculation agrees very well with the results of reference when the groove width is smaller and different from the instance of bigger groove width. Moreover, some characteristics of helical groove gyrotwt are discussed. The results shows that when the structure parameters (except groove width) are fixed, there is a best value of groove width to pitch, it can obtain relatively weak dispersion characteristics and big amplitude of transverse electric field. The helical groove structure can be used in the gyrotwt.