Graphene with 0.3% content was added to enhance the fracture toughness of Si3N4/SiC ceramics prepared by pressureless sintering. The microstructure, mechanical properties and toughening mechanism of ceramics were investigated. Results showed that the fracture strength significantly increased from 329.11 MPa to 443.19 MPa after adding graphene. The main toughening mechanism of ceramics was crack deflection since the existence of graphene platelets. The main phase compositions of samples were β-Si3N4 with a high aspect ratio and SiC located in the grain boundary, and the SiC particles could avoid the excessive growth of β-Si3N4 to reduce the strength.