TiO2 sample was prepared by sol-gel method from chloride titanium.The phase transformation of the prepared TiO2 sample was studied by in-situ XRD and normal XRD in different gas.The experimental results showed that the phase transformation temperatures of TiO2 were different under in-situ or normal XRD in different kinds of gas.The transformation of amorphous TiO2 to anatase was controlled by kinetics before 500 ℃.In-situ XRD showed that the growth of anatase was inhibited, but the transformation of anatase to rutile was accelerated under inactive nitrogen in contrast to air.Also better crystal was obtained under hydrogen than in argon.These all showed that external oxygen might accelerate the growth of TiO2, but reduced gas might partly counteract the negative influence of lack of external oxygen.The mechanism of phase transformation of TiO2 was studied by in-situ XRD in order to control the structure in situ.