Indium-gallium-zinc oxide (IGZO) photodetectors have been mostly studied in the ultraviolet region and rarely in the X-ray region. This study fabricates IGZO X-ray detectors on glass substrates using different post-deposition annealing (PDA) times. The photo-to-dark current ratio increases significantly from 2.6 to 392.3 after PDA because of a considerable reduction of the deep-level states. There are fewer residual electrons in the conduction band and recombination centers in the middle of the bandgap are eliminated. An IGZO X-ray detector with optimal PDA time has a sensitivity of <inline-formula> <tex-math notation="LaTeX">8.5\times 10^{-{3}}\,\,\mu \text{C} </tex-math></inline-formula>/(mGy<inline-formula> <tex-math notation="LaTeX">\cdot </tex-math></inline-formula>cm<inline-formula> <tex-math notation="LaTeX">^{{2}}{)} </tex-math></inline-formula> and a rise/decay time of 5.1/12.2 s with a bias of 10 V at a dose rate of 100 mGy/s. This result shows that IGZO is eminently suited to applications for X-ray detection.