Abstract: The join slurry is composed by silicon carbide(SiC) powder, phenolic resin and carbon black. Joining SiC green body after deairing, and then the reaction bonded SiC(RBSiC) joining mirror blank was prepared by reaction sintering. The relationship of the slurry formula and the microstructure of weld were discussed. The bending strength and the coefficient of thermal expansion of joining RBSiC ceramic were tested. The research shows that PEG200 and PVP can make carbon black dispersing homogeneous in the join slurry, and there is no residual carbon black in the weld after reaction sintering, so the microstructure of RBSiC ceramic is no different from the weld. The blending strength of joining green body and joining RBSiC ceramic are 24.9 MPa and 322.9 MP arespectively, and the fracture occurred in the base material. The CTE of base and weld material is only 0.011 4 ppm during -100-400 ℃.