Hafnium hydride (HfHx) is expected to extend the lifetime of control rod on fast reactors. Since hafnium has a large cross section for thermal neutron capture and hydrogen moderate fast neutron, hafnium is applicable to fast reactors by hydrogenating. Control rod worth is related to H/Hf ratio and so it is important for the lifetime of control rod to prevent releasing of hydrogen. Since the hydrogen dissociation depends on temperature, it is effective means for making dissociation smaller, that is, rise of HfHx pellet temperature is suppressed by filling the gap between cladding tube and HfHx pellet with sodium. The sodium filling procedure was tested with the parameter of gap width (0.16-0.36mm) and temperature (150-400 degree C). After filling sodium, the surface level of liquid sodium above HfHx pellet stack was measured by X-ray image intensifier. The result of preliminary tests shows that gas bubble remains in the gap. To vent gas from the gap, a process of vacuuming or tapping was added after the filling process. It was confirmed by checking the sodium level that no gas bubble remain in the gap. The filling test was also done for the cladding tube with the coating on inner surface of cladding, which prevents hydrogen from permeating through cladding tube. The similar result was obtained for the cladding with inner surface coating. The Sodium filling technique is important to the development of sodium bond type HfHx control rod. In this study, the sodium filling technique was established for short length tube (10cm) and it is necessary to check a usefulness of the technique for long length tube.