The polar and non-polar interfacesof grain boundariesin the zinc blendestructure canbedefined bythestoichiometryin theinterfaceregion,andit ispossibletoconstruct two polar interfaces and one non-polar interface for a symmetrical-tilt grain boundary for which the interface comprises the polar surfaces. The atomic and electronic structures of polar interfaces of the {122) 2 = 9 grain boundary in S i c have been examined using the selfconsistent tight-binding method coupled with the supercell technique, and they have been comparedwithourpreviousresultsfor thenon-polarinterface. In thepresentatomicmodels consistingofazigzagarrangement of5-7units. whichisconsistent withan ~n ~~i m a g e , o n e polar interlace contains the C--C wrong bonds and another polar interface contains the Si-Si wrong bonds, although the non-polar interface contains both types of wrong bonds. The present calculated results indicate that as well as the non-polar interface the respective polar interfaces are possibly stable without the generation of any macroscopic electrostatic fieldsor excess free carriers, although thewrong bondsintroducelocalizedstatesat the band edges and an increase in the electrostatic energy similar to the non-polar interface. This is because the atomicchargesin SiCare mainlycaused by the bond polarization and the wrong bonds do not generate any extra carriers or deep states in Sic, unlike ionic solids or heterovalent compound semiconductors. The calculated energy of formation of the pair of polar interfaces is rather smaller than that of the non-polar interfaces, because the energy increasecaused by bonddistortionsissmaller in the polarinterfaces.which containonlyone type of wrong bond. By using the calculated binding energies, the relative stability of the polar and non-polar interfaces of the (122) E = 9 boundary in Sic has been analysed thermodynamically as a function of the atomic chemical potentials. The calculated thermodynamic potentials of the interfaces indicate that either one or the other polar interlace is always more stable than the non-polar interface for the allowable ranges of the atomic chemical potentials. There is a general discussion of the stability of the polar interfaces in Sic and in compound semiconductors.