Abstract: The radial line TM01 mode, which is the dominant mode in the radial beam-wave interaction, determines the outcome of the interaction process. This paper presents the relative studies of radial line TM01 mode：the electromagnetic field distribution of radial line TM01 mode was obtained by theoretical derivation; then a new structure, exciting radial line TM01 mode by coaxial line TEM mode, was designed and verified by electromagnetic field simulation software. Results show that the conversion efficiency reaches 99% under the center frequency of 12GHz.