SiC-MgAl2O4 composites with SiAlON polytypoids (12H and 21R) as the bonding phases were prepared using SiC fine powder, MgAl2O4 fine powder and Al powder as raw materials. The effects of the firing temperature and the mass ratio of SiC to MgAl2O4 on the formation of SiAlON polytypoids as well as on the properties of samples were studied. The formation mechanism of SiAlON polytypoids was discussed. The results show that firing at 1 650 ℃ in the nitrogen atmosphere, SiC-MgAl2O4 composites with SiAlON polytypoids can be obtained. With the gradual increase of the mass ratio of SiC to MgAl2O4 in the raw materials, 12H forms first, and then coexists with 21H, finally, all 12H converts into 21R in samples. Both phases are plate crystals with flat and smooth edges. Compared with 21R, 12H is conducive to improving the density and the cold mechanical properties of materials. After firing at 1 650 ℃ in the nitrogen atmosphere, with the mass ratio of SiC to MgAl2O4 of 6:4, the sample has the cold modulus of rupture of 27 MPa, showing the optimal performance.