Distribution and dynamics of hydrogen atoms in the low-temperature phase of Mg2NiH4 have been studied by means of H-2 and H-1 NMR for Mg2NiD4 and Mg2NiH4, respectively. H-2 NMR spectra have been measured in the temperature range between 200 and 340 K, and the line shapes were simulated, The temperature dependence of H-2 NMR spectra was quite well simulated assuming a distorted tetrahedral configuration and a pseudoisotropic rotation of the NiD4 unit. The estimated jump frequency obeyed Arrhenius relation with a frequency factor of (0.8 +/- 0.6) x 10(13) Hz and an activation energy of 50.1 +/- 1.4 kJ/mol. H-1 NMR spectra were acquired from 240 to 360 K. The observed 1H second moments were 202 kHz(2) in the rigid lattice (240 K) and 46.6 kHz(2) in a motional state (360 K). The value in the rigid lattice supported the tetrahedron model, and the value in a motional state indicated the isotropic rotation of the NiH4 unit, Conclusively, the NiH4 unit has the distorted tetrahedral configuration and undergoes the pseudoisotropic rotation.