A dislocation model is proposed to explain the nucleation of domain walls under the influence of steady electric fields. The critical length of the nucleated domain wall along the surface, and the critical energy for nucleation can be calculated. It is found that the critical energy of nucleation varies inversely as (E-E_cr), where E is the applied electric field and E_cr, the critical field for nucleation which was earlier found to be approximately 8*10³V m^-1 for KNbO₃ single crystals.