The Kerr effect for a long polar polymer chain in a strong electric field is studied using the rotational-isomeric-state model on a tetrahedral lattice. A polymer chain containing only a longitudinal component of the dipole moment has been considered. In such chains the Kerr effect is the largest. The field dependence of conformational properties and of orientational characteristics has been calculated. The overall chain behavior in a strong electric field can be treated as a superposition of two effects: (1) an elongation of the "effective" rigid element of the chain (the rigid elements can be considered as;the trans sequences in the chain between gauche isomers); (2) an orientation of the "effective" segment like that in the freely jointed chain.