There is a continuous need to develop structural and functional components to sustain fusion plasma under the very severe environment such as intense radiation or high temperature in fusion reactors. The reference Si3N4 and novel Si3N4 based nanocomposites with carbon nanotube, graphene or carbon black additions were irradiated in a controlled-temperature irradiation rig inside the Budapest Research Reaktor (BRR) at a temperature of 270 °C. In the irradiation channel of BRR the 1,5×1013 n/cm2 fluence can be reached in 700 hours. The effect of the neutron radiation on the mechanical properties of Si3N4 based nanocomposites with different carbon additives was investigated. In the case of 3-point bending test graphene and carbon nanotube added samples showed an increase in strength ~100-300 MPa after irradiation. 4-point bending strength measurements resulted in a significant increase of strength in the case of reference samples ~200-400 MPa and carbon nanotube added composites ~200-300 MPa after irradiation. Other samples showed no change in strength after irradiation.