The electron emission properties of hafnium nitride field emitter arrays with 10 000 emission tips in the triode configuration were investigated at room temperature, 100 °C, 200 °C, and 300 °C. After cooling the sample to room temperature, the same measurements were repeated again. In the first high temperature test, the current-voltage curve at room temperature and 100 °C did not show a significant difference, but it shifted to a lower voltage for temperatures higher than 200 °C. In the second high temperature test, the current-voltage characteristics were quite similar to those obtained in the first high temperature test, except for the fact that the current-voltage curves at room temperature and 100 °C were lower than those in the first high temperature test. A Seppen-Katamuki analysis revealed that the emitter surface at temperatures less than 200 °C had a work function of about 4.3 eV once they had been heated up higher than 200 °C. At 300 °C, both for the first and second high temperature tests, the work function of the emitter was estimated to be approximately 3.1–3.3 eV, which was lower than the value expected from our previous study. The validity of the estimated work functions was confirmed by the fact that the experimentally obtained enhancement of the current and transconductance agreed with the values calculated from the theory using the estimated work function. Arguments were made for different situations such as changes in the apex radius or in a number of active tips.