The fabrication of dense Si3N4 nano-ceramics via a high-energy mechanical milling process and subsequent spark plasma sintering (SPS) was reported in this paper. A thermodynamically stable β-Si3N4 powder mixed with 5-mol% Y2O3 and 2-mol% Al2O3 was used as starting powders for high-energy mechanical milling. Milling resulted in the significant decrease of crystalline size and the amorphization of the starting powders. Based on XRD measurements, the amount of the amorphous phase and the grain sizes of the remaining crystalline silicon nitride depended on the charge ratio. Milled powders could be SPSed at relatively low temperature. A homogenous dense silicon nitride ceramics with grain diameter of about 70 nm could be fabricated through SPS at a low temperature of 1600oC for 5 min. The significant decrease of grain size greatly increased the hardness of obtained ceramics.