As a study of the mechanism of hydrogen absorption into promising hydrogen storage materials,two ZrNiAl type uranium intermetallic compounds,UNiAl and UPtAl,were investigated with the ab initio calculation as well as the experiments regarding their hydrogen absorption properties.From the calculation,it was shown that for UNiAlHx hydride the transformation from Type A structure(part of Ni atoms being distributed on U plane)to Type B(all Ni atoms distributed on U plane)would occur as observed experimentally on account of decrease of energy of hydride with increasing the hydrogen content,while for UPtAlHx such a transformation would not occur due to increase of energy of hydride with increasing the hydrogen content,preventing a large amount of hydrogen to be absorbed in the latter case.In the former case,movement of Ni atoms toward c-axis was attributed to the shrinkage of c-axis,which would cause increase in the hydrogen absorption capacity,while in the latter case such movement of Pt atoms was not confirmed to take place.