In order to realize thermoelectric power generation using high temperature waste heat, oxides are the strongest candidates for thermoelectric materials. So far, p-type oxides with good thermoelectric properties have been reported in Co-based oxides. On the other hand, n-type oxides whose properties are as good as p-type oxides have never been discovered. It is found out using a high throughput screening technique that Ln-Ni-O (Ln : lantahnoid) systems show n-type thermoelectric properties. LnNiO₃ and Ln₂NiO₄ are mainly formed in samples possessing the n-type property. Seebeck coefficient of LnNiO₃ and Ln₂NiO₄ (Ln : La or Nd) is -20--40 mV/K at 973 K. While electrical resistivity of LaNiO₃ is as low as 1.4 mW cm, both Nd-Ni-O shows high r of 20-50 mW cm at 973 K in air. At present, the highest power factor of Ln-Ni-O systems is about 0.1 mW/mK² at 973 K for LaNiO₃. Substitution of Ln with other cations seems to be effective to enhance power factor. Optimizing substituted elements and compositions is being tried using the high throughput screening technique.