H2S gas sensor based on α-Fe2O3 nanoparticles was fabricated by post-thermal annealing of Fe3O4 precursor which was synthesized using a facile hydrothermal route. The characteristic techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were adopted to characterize the chemical composition and microstructure of the obtained samples. Gas-sensing performance of the sensor was investigated at different operation temperatures from 100 °C to 400 °C. Results showed that the sensor exhibited the best sensitivity, reproducibility and long-term stability for detecting H2S gas at an operating temperature of 300 °C. The detection limit towards H2S gas was 0.05 ppm, and the response time and recovery time was 30 s and 5 s, respectively. In addition, sensing mechanism of the sensor towards H2S was discussed.