The bulk modulus B-o = 290(5) GPa and its first pressure derivative B-o' = 4.9(6) were obtained for c-Si3N4 from volume versus pressure dependence. Measurements were performed under quasi-hydrostatic conditions in a diamond anvil cell to 53 GPa using synchrotron radiation and energy dispersive X-ray powder diffraction. This combined with nanoindentation measurements determined the shear modulus G(o) of c-Si3N4 to be 148(16) GPa. The Vickers microhardness H-V(0.5) for dense, oxygen-free C-Si3N4 was estimated to be between 30 and 43 GPa. Both the elastic moduli and microhardness of c-Si3N4 exceed those of the hexagonal counterparts, alpha- and P-phases.The bulk modulus B, = 290(5) GPa and its first pressure derivative B'(0) = 4.9(6) were obtained for c-Si3N4 from volume versus pressure dependence. Measurements were performed under quasi-hydrostatic conditions in a diamond anvil cell to 53 GPa using synchrotron radiation and energy dispersive X-ray powder diffraction. This combined with nanoinclentation measurements determined the shear modulus Go of c-Si3N4 to be 148(16) GPa. The Vickers microhardness Hv(0.5) for dense, oxygen-free c-Si3N4 was estimated to be between 30 and 43 GPa. Both the elastic moduli and microhardness of c-Si3N4 exceed those of the hexagonal counterparts, alpha- and beta-phases.