We theoretically studied the influence of the applied electric filed on thermoelectric properties of intrinsic bismuth telluride nanowires, which are grown along [110] direction. The electronic structure and wave functions were calculated by solving the self-consistent system of the Schrodinger and Poisson equations by means of both the linearization of Poisson's equation and the spectral element method. The thermoelectric parameters were calculated using a constant relaxation-time approximation. The external electric field can increase the Seebeck coefficient of a 7- and 15-nm-thick square nanowire by nearly a factor of two, and enhance the figure of merit by an order of magnitude.