In this paper, the ZnS nanoparticles were synthesized according to the process conditions of hydrothermal synthesis. When the molar ratio of Zn to S was 1:1.2, it was confirmed that it had a cubic single phase and a high crystal phase. After the molar ratio is fixed, hydrothermal synthesis was conducted at 180℃ for 24, 36, 72 and 96 h in order to confirm the structural change with the change of hydrothermal synthesis times. As the hydrothermal synthesis times increased, the particle size increased. The hydrothermal synthesized particle size for 72 h was considered to be suitable for sintering. The ZnS ceramic had a density of 99.7% and an excellent transmittance of ~70% in the long-wavelength region.