Abstract: Varying the span of each cavity from uniform to nonuniform, a new device called nonuniform threecavity transittime oscillator (NTCTTO) has been developed. By employing a SUPERFISH code, the high frequency features of a nonuniform threecavity resonator were investigated numerically. The influences of span change on TM0102π/3like mode and the corresponding frequency, on the zaxial component of the electric field and its radial distribution were studied. Results show whether the TM0102π/3like mode exists depends on the span of each cavity. When the TM0102π/3like mode exists, the influence of the span change on the corresponding frequency is small(the difference between the highest and the lowest frequency is less than 4%); when the first cavity gets longer, the zaxial com