By using the method of point relation for identification of SiC polytype, more than 1100 Laue patterns of SiC single crystals were analysed and 61 kinds of new modifications of α-SiC were found, in which the largest lattice parameter of c-axis in the hexagonal unit cell is 2622.8?. So far the number of SiC polytypes reaches 110 kinds, in which the hexagonal polytypes are of 30 kinds and the rhombohedral polytypes are of 80 kinds.According to the experimental data, the three different mechanisms of forming the SiC polytypes at the present time were discussed:1) the screw dislocation theory,2) the growth condition theory,3) the thermodynamic theory.