Coaxial slow wave structure (SWS) is the most important part in MILO. According to the Maxwell equations and Floquet theorem, the dispersion relation of several TM0 n modes in MILO coaxial SWS is educed in this paper. Through numerical calculation, the impacts of several structure parameters of MILO coaxial SWS on its high frequency characteristic are analyzed, and the results are useful to the design of MILO.