In this work, a single-crystalline β-Ga2O3 epilayer was grown on (0001) sapphire at low temperature by low-pressure metal organic chemical vapor deposition. The optimized parameters for the chamber pressure, oxygen flow, and growth temperature were 15 Torr, 200 sccm, and 500 °C, respectively. The β-Ga2O3 epilayer was fabricated as a metal-semiconductor-metal solar-blind deep ultraviolet photodetector. Due to the gallium oxide grown at low temperature, the as-grown β-Ga2O3 epilayer was annealed at 800 °C in atmosphere or in a nitrogen environment. The effects of defects of the β-Ga2O3 epilayer before and after N2 annealing were studied using x-ray diffraction system, cathodoluminescence at differential temperature, and Hall measurement. The β-Ga2O3 epilayer that was N2 annealed for 15 min presented better photodetector performance than the as-grown β-Ga2O3 epilayer. The annealed epilayer exhibited a dark current of 1.6 × 10−13 A under 5 V bias.