A nano-Si3N4/5 wt% nano-C composite was successfully fabricated for the first time via high-energy ball milling, followed by spark plasma sintering. The milling promoted the amorphization of the starting powders; most of the carbon particles were transformed into nano-size and embedded in the amorphous phase. This, combined with a low sintering temperature and a rapid densification rate, prevented the reactions between carbon and the other starting powders, leading to a uniform nano/nano-composite microstructure. Nano-sized carbon grains with an average diameter of about 10 nm were homogeneously dispersed in nano-sized (about 70 nm) Si3N4 grain boundaries. The hardness of the obtained nano-ceramics is comparable with that of conventional Si3N4, whereas Young's modulus is significantly decreased.