Lanthanium is added to NiAl which is a promising coating material for the wet-seal area of the separator in molten carbonate fuel cells. The effect of lathanium on the corrosion of NiAl in molten carbonate is investigated. Lathanium is precipitated at the grain boundaries of NiAl and reduces the grain size by one-fifth to one-eighth when added above 1.0 at.% by arc-melting. Though precipitated lathanium shows reactive effects and slightly increases the adherence of the oxide layer in cyclic oxidation. the oxide layer is disrupted at the grain boundaries when corroded in molten carbonate salt for more than 100 h. This behaviour is due to the internal stress which results from the oxidation.