A low electric field applied during film deposition has a significant influence on the orientation of ferroelectric thin films prepared by pulsed laser deposition. C-axis oriented growth of LiNbO3 films was demonstrated on fused silica with the aid of a low bias voltage Vb50-120 V, and complete c-axis orientation was achieved at Vb5110 V. In contrast, the electric field cannot induce c-axis orientation of LiTaO3 films. Theoretical analysis suggests that the electrostatic energy provides an extra driving force for the c-axis oriented growth of the ferroelectric films if the permittivity components satisfy e33/e11<2, as in LiNbO3 film, but not if e33/e11.2 at the deposition temperature, as in LiTaO3 film.