Annealing can usually inhibit the formation of an unstable brookite mesophase in the crystal structure, enhancing the growth of an anatase titanium dioxide (TiO2) thin film. In this study, after a TiO2 solution was prepared with and without a small amount (1 wt%) of the organic solvent dimethylformamide (DMF), TiO2 films were obtained by dynamic spin-coating and were annealed at temperatures of 300, 500, and 700 °C. The structural and optical properties as well as the surface morphologies of the films were studied. The experimental results show that the TiO2 film annealed at 500 °C exhibits a polycrystalline anatase and brookite multiphase and usefully inhibits the formation of the brookite mesophase when annealed at 700 °C. Moreover, adding the DMF effectively contributes to the formation of the polycrystalline phase at 300 °C. The compactness of the annealed TiO2 film was improved by adding 1 wt% DMF, as shown by topographic images obtained by atomic force microscopy, leading to decreased transmittance and increased absorbance. The band gap (Eg) values of all the samples were also estimated. The variation of Eg was evidenced by the structural change and the redshift of the absorption edge.