Single crystal 4H and 6H polytypes of SiC have been deformed in compression at 1300 degreesC. Using two-beam bright-field and weak-beam dark-field techniques of TEM, the stacking fault energy of these two SiC polytypes has been determined to be 14.7+/-2.5 mJ/m(2) (4H-SiC) and 2.9+/-0.6 mJ/m(2) (6H-SiC). The experimental values of stacking fault energy for 4H- and 6H-SiC have been compared with estimates obtained from a generalized axial next-nearest-neighbor Ising (ANNNI) spin model. I is found that the theoretical models predict the lower stacking fault energy of 6H-SiC compared to that of 4H-SiC, and the predicted energies are, respectively, within 5% and 40% of the experimental values.