A high-energy diffraction technique is presented that uses synchrotron X-rays to characterize the in situ deformation response of pressurized creep tubes at elevated temperature. In addition to the X-ray diffraction measurement, the technique allows the macroscopic creep strain to be measured simultaneously during X-ray exposure. We demonstrated this technique in two areas at different temperatures in the tube specimen. From the X-ray diffraction patterns, we obtained a typical creep curve with identifiable secondary and tertiary creep response at the high temperature area, and only observed the secondary creep response at the low temperature area. The diffraction peak broadening analysis directly showed the development of the dislocation structures and lattice strain during deformation and make it possible to track the development of creep void nucleation, growth and coalescence.