Theoretical description of the Pockels effect is offered in which statement of a problem in the form of Maxwell equations allows to go on to wave equations directly and to find their solutions. Analytical expressions determining phase velocities and polarization of the optical plane waves, propagating in crystal of lithium niobate in principal crystallographic directions, for different cases of influence of an exterior static electric field are gained. Appropriate conclusions about optimal use of the Pockels effect for process control of the optical modulation device, in particular on a crystal of lithium niobate, are performed.