The nitrogen absorption and desorption property of LaNi 5 was investigated using NH 3 and N 2 as nitrogen sources. It was found that both as purchased and milling processed LaNi 5 powder can absorb and desorb nitrogen using NH 3 as the nitrogen source, while milled LaNi 5 powder can absorb and desorb more. XRD, cross sectional SEM and EPMA were performed to elucidate the nitrogen stored phases. When N 2 gas was used as the nitrogen source, it was found that the amount of absorbed nitrogen significantly decreased. By using high pressure for the absorption treatment or performing an activation treatment using hydrogen absorption/desorption before the nitrogen absorption treatment, milled LaNi 5 was found to absorb nitrogen of 0.1mol/mol Finally, the mechanism of the nitrogen absorption was discussed from the EPMA and kinetic data of the nitrogen absorption, and it was suggested that the mechanism of the nitrogen absorption might be explained by a combined model of a diffusion controlled process and an interface controlled process.