The large scale light-weight silicon carbide(SiC) mirror blanks at φ620 mm and 700 mm×300 mm were prepared with gel-casting method.After drying,burning out the organic elements,infiltrating carbon and reaction-sintering,the reaction-bonded SiC (RB-SiC) mirrors were fabricated.The results of ground mirrors show that the structure of RB-SiC is full solid,and the mechanical and thermal properties of RB-SiC are excellent in bending strength of 350 MPa,fracture toughness of 4.1 MPa·m^1/2 and coefficient of thermal expansion (CET) of 2.67×10^-6/K.The surface roughness (RMS) of the polished mirror is better than 3 nm,it is a fine material for the space-borne large scale light-weight mirror.