Chemical vapor-deposited amorphous Si_3N_4 (CVD-amorphous Si_3N_4) up to 4.2mm in thickness has been prepared from a gaseous mixture of NH_3 and H_2-carried SiCl_4 under various deposition conditions. The formation of the CVD-amorphous Si_3N_4 depended strongly on the deposition temperature, total gas pressure and gas flow rate. The CVD-amorphous Si_3N_4 prepared at 1100-1300℃ does not crystallize by heating at each deposition temperature. Their density and deposition rate are markedly dependent on deposition conditions and have maximum values of 3.00g/cm^3 (94% of the theoretical density of α-Si_3N_4) and 0.36mm/hr, respectively. The Vickers microhardness of the CVD-amorphous Si_3N_4 at room temperature varies between 2200 and 3200kg/mm^2 according to its deposition conditions. The hardness at 1300℃ is 1200～1300 kg/mm^2. The thermal conductivity was 0.010cal/cm/sec/℃ at 20℃ and 0.012cal/cm/sec/℃ at 1300℃. The thermal expansion coefficient at 20～1200℃ is 2.99±0.05/℃. The formation mechanism and the effect of gas flow patterns on the deposition rate of the CVD-amorphous Si_3N_4 are also discussed
