Phase stability and effects of sintering atmosphere on the crystalline structure of Np-doped yttria-stabilized zirconia (YSZ) were evaluated in comparison with those same properties and conditions for Ce-doped YSZ. Different sintering atmospheres for Ce-doped YSZ led to differences in phase formation through reduction of the dopant from Ce4+ to Ce3+ On the other hand, YSZ specimens containing up to 40 mol% Np formed only a fluorite-structure phase regardless of sintering atmosphere. Yttria-stabilized zirconia thus seems to accommodate Np within a wide range of concentrations and to have excellent phase stability under both oxidizing and reducing atmospheres.