There has been a controversy on the formation mechanism of TiO2 nanotubes. This study was conducted to elucidate the formation mechanism of TiO2 nanotubes. TiO2 nanotubes were prepared by a hydrothermal method. TiO2 nanotubes formation mechanism was investigated by controlling the formation time. It was found that TiO2 nanotubes were formed by growing, not by wrapping of sheets. The phase structure of hydrogen titanate nanotubes was different from that of TiO2 nanotubes. It is important to wash the sodium titanate nanotubes with an acidic solution to get hydrogen titanate nanotubes and then to calcine the hydrogen titanate nanotubes around 400℃ to obtain TiO2 nanotubes.