The authors developed a new analysis technique for atomic disorder structures in full-Heusler alloys using x-ray diffraction (XRD) with Co Kα and Cu Kα sources. The technique can quantitatively evaluate all the atomic disorders for the exchanges between X, Y, and Z atoms in full-Heusler X2YZ alloys. In particular, the technique can treat the DO3 disorder that cannot be analyzed by ordinary Cu Kα XRD. By applying this technique to full-Heusler Co2FeSi alloy thin films formed by rapid thermal annealing (RTA), RTA-temperature (TA) dependence of the atomic disorders was revealed. The site occupancies of Co, Fe, and Si atoms on their original sites were 98%, 90%, and 93%, respectively, for the film formed at TA=800 °C, indicating that the RTA-formed Co2FeSi film had the L21 structure with the extremely high degree of ordering.