A large-signal computer simulation was made for several simplified nonuniform doping profiles that correspond to those of practicalX-band Gunn diode. Maximum available dc-to-RF conversion efficiency was searched for each profile by varying the oscillation frequency and the magnitude of the RF terminal voltage while the dc bias was fixed. The result showed that a higher-resistance cathode-side region degrades the efficiency mainly through decrease of the amplitude of the RF current, while a higher resistance anode-side region degrades the efficiency much more by delaying the phase of the domain nucleation. An improvement of the efficiency by about 20 percent was found to be possible by introducing a low-resistance region near the anode to deform the RF current waveform. Computations were also made for several nonuniform profiles that were observed experimentally. Calculated and experimental results were compared, and reasonable agreement between the two was found, though there were some cases in which the agreement was not obtained due to nonideal properties of electrode boundaries.