Thin films of tungsten oxide with different crystallization temperatures were synthesized by the sol–gel method and used to fabricate photodetectors. The surface morphology, microstructure, and crystal structure of the films were analyzed by scanning electron microscopy, X-ray diffraction spectroscopy, and photoluminescence spectroscopy, respectively. The effects of annealing temperature and grain size of a grain on the structure and optoelectrical properties were examined. The responsivity at 410 nm and the rejection ratio for the 600 ℃-annealed W18O49 film were 0.0268 A/W and 407.8, respectively. Meanwhile, the relationships among thin films were also investigated and discussed. A photo-to-dark current contrast ratio of 6.5 × 102 was achieved. I–V measurement also indicated that the optimum annealing temperature for the substrates was 600 ℃.