Structural parameters of Pr3MgNi14 after a cyclic hydrogen absorption-desorption process were investigated by X-ray diffraction. Pr3MgNi14 consisted of two phases: 80% Gd2Co7-type structure and 20% PuNi3-type structure. The pressure-composition (P-C) isotherm of Pr3MgNi14 indicates a maximum hydrogen capacity of 1.12 H/M (1.61 mass %) at 298 K. The cyclic property of Pr3MgNi14 up to 1000 cycles was measured at 313 K. The retention rate of the sample was 87.5% at 1000 cycles, which compares favorably with that of LaNi5. After 1000 cycles, the expansions of lattice parameters a and c and the lengths along the c-axes of the PrNi5 and PrMgNi4 cells of the Gd2Co7-type structures were 0.20%, 1.26%, 0.47%, and 3.68%, respectively. The metal sublattice expanded anisotropically after the cyclic test. The isotropic and anisotropic lattice strains can be refined by Rietveld analysis. The anisotropic and isotropic lattice strains were almost saturated at the first activation process and reached values of 0.2% and 0.1%, respectively, after 1000 cycles. These values are smaller by 1 order of magnitude than those of LaNi5.