Carbothermal reduction and nitridation synthesis of silicon nitride was conducted at 1 300～1 600 ℃ in nitrogen or nitrogen-hydrogen mixture. Fumed silica was mixed with graphite using molar ratio of SiO2∶C=1∶4.5. Powder samples were pressed into pellet before reaction. CO evolution in the process was monitored by an infrared sensor. The oxygen， nitrogen， carbon content of reacted sample was analysed by LECO element determinators. XRD was used to identify phases formed in the reacted sample. Reduction of SiO2 commenced below 1 300 ℃. The reduction rate increased with increasing temperature， but slowed down when temperature was higher than 1 570 ℃， since the residual reactant was covered by product. Due to the small difference of the equilibrium CO partial pressure between Si3N4 and SiC formation， the boundary temperature was not obvious. The mechanism of carbothermal reduction/nitridation synthesis of silicon nitride needs further investigation.