Nitride fuel is a promising nuclear fuel in fast breeder reactor (FBR) or accelerator-driven subcritical reactor (ADSR) system. In this study, high-density UN pellets were prepared by Spark plasma sintering (SPS) technique. The sample density strongly depended on the sintering temperature and pressure, and the pellets with 90% of theoretical density were easily obtained with low sintering temperature and short sintering time without any milling process. The grain size and pore size were much smaller compared with those for samples prepared by conventional sintering process. Despite of the small grain size, the thermal conductivity remains the high value. The SPS process permits easy densification of nitrides without any deterioration of thermal and mechanical properties, considered to be suitable as a preparation method of nitride fuels.