Colloidal crystals composed of polystyrene micro-spheres (negatively charged, with a diameter of 300 nm) are fabricated by injecting aqueous suspensions of the micro-spheres with different volume fractions into sample cells which are made by gluing together two glass plates. Whose surfaces are coated with conductive films. The colloidal crystals have a face-centered-cubic structure with their (111) planes parallel to the surface of the sample cells. Laser diffraction is used to measure the structural changes of the colloidal crystals in an electric field. Structures of the colloidal crystals are characterized by using the Kossel-line method. It is found that the colloidal crystals are compressed isotropically in the electrical field. The lattice constants of the colloidal crystals decrease with the increase of the electric field, maintaining their face-centered-cubic structure. Results can be explained by the combined action of the electric field force, electrostatic repulsion and electrohydrodynamic force. The electric field makes all of the micro-spheres migrate to the positive plate of the sample cell and leads to a compression in the direction along the electric field. Then the electrohydrodynamic force produces an attractive interaction between the micro-spheres in the direction perpendicular to the electric field.