Like atoms and molecules with directional interactions, anisotropic particles could potentially assemble into a much wider range of crystalline arrays and meso-structures than spherical particles with isotropic interactions. In this paper, the electric-field directed assembly of geometrically anisotropic particlescolloidal dimers is studied. Rich phase behavior and different assembly regimes are found, primarily arising from the broken radial symmetry in particles. The orientations of individual dimers depend on the frequency of the electric field, the ramping direction of frequency, and the salt concentration. The competition and balance between the hydrodynamic, electric, and Brownian torques determine the orientation of individual particles, while the competition between the electrohydrodynamic force and dipolar interaction determines the aggregation of aligned particles at a given experimental condition. The field distribution near the electrode is critical to understand the orientation and assembly behavior of colloidal dimers on a conducting substrate. This study also demonstrates the effectiveness, the reversibility, and potential opportunity of applying electric field to control the orientation and direct the assembly of non-spherical particles. In particular, two dimensional close-packed crystals of perpendicularly aligned dimers are obtained, which shows promise in fabricating 3D photonic crystals based on dimer-like colloids and field-directed display.