Pyrolytic carbon-silicon nitride ceramics with dense silicon nitride coating (PyC-Si3N4/Si3N4) were fabricated by chemical vapor infiltration of PyC into porous Si3N4 ceramics combined with chemical vapor deposition of Si3N4 at their surface. The electromagnetic shielding effectiveness (SE) of PyC-Si3N4/Si3N4 increases with the increase of PyC content and specimen thickness. When the content of PyC is 11.7 vol% and the thickness of specimen is 2.8 mm, the mean total SE of PyC-Si3N4/Si3N4 reaches 43.2 dB in the frequency range of 8.212.4 GHz. Due to the excellent sealing effect of dense Si3N4 coating, the mean total SE of PyC-Si3N4/Si3N4 remains unchanged even after oxidation at 1100 degrees C for 10 h in air.