We prepared bulk nanocrystalline SmCo6.6Nb0.4 sintered magnet material by spark plasma sintering technique. X-ray diffraction patterns show that the magnet exhibits a stable TbCu7 structure. Transmission electron microscopy indicates that the microstructure of the magnet is composed of SmCo6.6Nb0.4 single-phase grains with an average grain size of 30 nm. Magnetic measurement shows that under a 7 T magnetic field, the coercivity of the magnet reaches as high as 2.8 T; the saturation magnetization and the remanence are 69.6 and 51.4 emu/g, respectively. The magnet exhibits good thermal stability with the coercivity of 0.48 T at 773 K, and the coercivity temperature coefficient beta of -0.169%/K.