In this paper, we presented a beam adjustable antenna made up of a slit waveguide and a dielectric slab. In order to change the radiation main-beam angle, we changed the phase constant of the waveguide mode by inserting a dielectric slab for perturbing its field distribution. The direction of radiation main-beam can be steered by dynamically changing the position of the dielectric slab. For the theoretical analysis, the dispersion relation, including the phase and attenuation constants, was determined by solving the transverse resonance equation. An agreement between the theoretical and experimental radiation pattern verifies the beam-steering mechanism. Up to 23beam-steering angle can be achieved using this approach.