All-solid-state sodium batteries with high safety and high energy density are expected as next generation rechargeable batteries. Active materials play the key role to develop the capacity and the reversibility of the batteries. We have focused on the transition metal sulfides as the active materials. We have reported that the all-solid-state cells using TiS2 operated reversibly[1]. In term of the practical use, the further increase of the capacity is important. One of the solutionsll is the application of sodium rich transition metal sulfides as active materials. In previous reports, the several new crystal polymorphs of Na2TiS3 were produced by melt quenching and mechanochemical reaction [2, 3]. P2-type layered Na2TiS3 obtained by melt quenching showed twice higher capacity than that of the stable phase of O3-type layered Na2TiS3. Cubic rocksalt phase Na2TiS3 prepared by mechanochemical process was also a metastable phase with high capacity. The metastable phases with high ionic conductivity are advantageous for insertion/extraction. Sodium niobium sulfides have sodium-rich compositions such as Na3NbS4. In lithium batteries, cubic rocksalt Li3NbS4 synthesized by mechanochemical process showed the reversible capacity of 400 mAh g-1 [4]. In this study, we have synthesized Na3NbS4 and optimized their crystal polymorphs by melt quenching and mechanochemical process for high capacity electrode.Electrode materials with the composition of Na3NbS4 were prepared by solid phase reaction or mechanochemical reaction of Na2S, NbS2 and S. DTA curves of the mixture of Na2S, NbS2 and S with the Na3NbS4 composition suggested that phase transition occurred at 630oC. XRD patterns of Na3NbS4 prepared by melt quenching from 700oC showed that O3-type layered phase were obtained. The all-solid-state cells using the layered Na3NbS4 as active materials showed 80 mAh g-1, suggesting that the O3-type sturucture was not suitable for sodium extraction/insertion. On the other hand, the mechanochemical process produced the new metastable phase of Na3NbS4. The all-solid-state cells using the new phase showed the increment on the capacity of 240 mAh g-1. The new crystal polymorph of Na3NbS4 has also been profitable for sodium extraction/insertion. The newly developed Na3NbS4 is a promising active material with high capacity for all-solid-state sodium secondary batteries. Reference s : [1] A. Hayashi, K. Noi, A. Sakuda and M. Tatsumisago, Nat. Commun., 3 (2012) 856.[2] A. Nasu, M. Otoyama, A. Sakuda, A. Hayashi, and M. Tatsumisago, Chem. Lett., 48 (2019) 288.[3] A. Nasu, M. Otoyama, A. Sakuda, A. Hayashi, and M. Tatsumisago, J. Ceram. Soc. Jpn., 127 (2019) 514.[4] A. Sakuda, T. Takeuchi, K. Okamura, H. Kobayashi, H. Sakaebe, K. Tatsumi and Z. Ogumi, Sci. Rep., 4 (2014) 2.