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We report on the current transport mechanism in Au/GaN nanowire–Schottky diodes at high temperature using current–voltage characterization in the temperature range of 300–573 K. We observed that the ideality factor for the GaN nano-Schottky diodes decreased from 11.2 at 323 K and reached its lowest value of 7.9 at 500 K. Then, it started to increase with increasing temperatures up to 573 K. On the other hand, the Schottky barrier height increased throughout all of the temperature ranges up to 573 K. We found that the abnormal behaviors of the barrier heights and the ideality factors in the current transport by varying the temperature of 300–500 K can be well explained by a combination of the thermionic field emission (TFE) mechanism and a model with a Gaussian distribution of the barrier heights that were predominant at the interface between the Schottky metal (Au) and the semiconductor (GaN nanowire). We also suggested that the increase of the ideality factor at a temperature higher than 500 K could be attributed to a trap-assisted tunneling current in a TFE mechanism.