One-dimensional TiO2 nanostructure synthesized using a hydrothermal method showed significant differences in the bandgap energies and crystal phase properties under different synthesis conditions. The objectives of this study were to evaluate the effect of the hydrothermal reaction conditions on the bandgap energy and to optimize the reaction conditions using a 3-factor 3-level Box Behnken design. The factors investigated include temperature, NaOH concentration and TiO2 concentration (BBD). A quadratic response surface model predicted optimum conditions for the maximum and minimum bandgap energy. The BBD analysis and other statistical tools provided evidence showing the temperature and NaOH concentration significantly affected the 1D TiO2 bandgap energy. Pure anatase was associated with a higher bandgap energy, whereas the biphasic anatase-rutile and anatase-TiO2-B phases were linked with decreasing the bandgap energy. In addition, the crystal size and specific surface area (SSA) impact on the bandgap energy was due to the size quantization effect.