Abstract: This paper presents the study on the coaxial diskloaded waveguide as the slowwave structure of the relativistic traveling wave tube(RTWT). The inner conductor is employed in this structure to improve the characteristics of such slowwave system. The paper emphasizes the smallsignal analysis of the RTWT with coaxial diskloaded waveguide, and discusses the influence of the beam parameters and the structure dimensions on the smallsignal gain and phase velocity of slowwave, the results provides the theoretical basis for the design of the coaxial diskloaded RTWT.