Abstract: The tilt of inner conductor unavoidably occurs in coaxial gyrotrons. The effects of tilted inner conductor on eigenvalue, quality factor Q, resonant frequency, transverse electric field, mode competition and electronic efficiency are presented. The theory is illustrated in a 170 GHz TE_{31, 12} corrugated coaxial-gyrotron. The results indicate that eigenvalue and quality factor Q increase slightly when the tilt angle θ of inner conductor increases. However, within the range of 0-0.5°, the electronic efficiency decreases slightly when θ grows. If θ increases to 1.3°, the efficiency of beam-wave interaction reduces to 5% because mode competition becomes serious and the transverse electric field is distorted severely. Effected by the tilted inner conductor, the resonant frequency of cavity increases slightly with θ rising.