A halogen-and acid-free aqueous solution was developed to crystallize anatase TiO2 nano-particles. Spinal or zigzag chains of TiO6 octahedrons were formed from [TiO(C2O4)2] 2-ions. They were then gradually transformed into rutile TiO2 in the solutions. Rearrangement of octahedrons realized aqueous phase transition from anatase to rutile.