The phase transformation of Se/(Cu,In,Ga)/Mo/glass thin films during annealing in a vacuum on and off state was studied in a real-time synchrotron X-ray scattering experiment. The crystalline CIGS phase is a solid solution of crystalline CIS and CGS phases. The crystalline CIS phase was formed first at lower temperature. By increasing the temperature, the crystalline CIS phase disappeared, while the crystalline Ga2Se3, In2Se3, Cu2In phases grew simultaneously. Finally, the crystalline CIGS phase was formed at higher temperature, while the crystalline Ga2Se3, In2Se3, Cu2In phases disappeared gradually. The behavior of the crystal domain sizes was consistent with the changes of X-ray powder diffraction profiles. The high crystallization temperature of the CIGS phase was attributed to the activation energy barrier for the diffusion of Ga ions into the intermediate CIS phase.