Abstract: The decomposition of the hydride in cold-roiled nickel is investigated by doppler broadening and X-ray method. The experimental results show that when cold-rolled nickel is charged with hydrogen in a 0.5mol/l H₂SO₄+ 250mg/l As₂O₃ solution, a fcc β phase (hydride) is formed. It is unstable and decomposes immeditely after the interuption of the charging process. Hydrogen is not sensitive for positron, but if low hydrogen concentration is introduced into the cold-rolled nickel whinin short charging time. PAT can detect the process of the hydride decomposition. Its results are consistent with the outcome using X-ray method. A mechanism of interaction between hydrogen and vacancy has been suggested, hydrogen atoms can be regarded as the centres for vacancy agglomeration, resulting in the formation of vacancy clusters. It may explain the variation of S-parameter with ageing time when the nickel hydrides decompose.