[en] Neutron diffraction data of palladium electrode in heavy water electrolysis cell have been collected under the balance condition of absorption and evolution of deuterium. The analysis results show that after palladium is charged with deuterium, the original α-phase disappears thoroughly and the sample has a transition from α-phase into β-phase and a new α'-phase coexisting with the β-phase is also observed. In the β-phase of PdD_x system deuterium atoms enter not only the octahedral sites but also the tetrahedral ones of the Pd face-centered-cubic lattice. The occupancy of deuterium atom in above-mentioned two types of sites is 0.3 and 0.2 respectively.From this, x is determined to be 0.7