Abstract: In order to investigate the highfrequency characteristics of the Xband fiveunit transittime tube oscillator (TTTO), the method of finiteunit differential of time domain (FDTD) is involved, then the Maxwell Equations are solved. And the frequency of the eigenmodes (belonging to nonpi mode of TM01 mode) and its distribution of electric field together with the Qvalue in the cavity are presented. Additionally, the experimental results are also put out. The experimental results are in accordance with the results of the numerical solution, which proves the reliability of the results.