The design and implementation of a reflectarray with a single-layer perforated dielectric substrate is presented. The perforated dielectric layer, used as the reflecting surface of the proposed antenna, is realized by drilling air holes with different diameters on the dielectric substrate. Thus, the effective permittivity of the dielectric substrate is altered by drilling holes with different diameters, and then the substrate is equivalent to an inhomogeneous dielectric layer that can be controlled by these holes to collimate the reflected waves in the special direction. The reflectarray is composed of 29 × 29 elements, and it covers an area of 24.65 × 24.65 cm2. Varying the hole's diameter can lead to 360 phase shift. The reflectarray is offset fed by a linearly polarized pyramidal horn antenna. Full-wave analysis software CST Microwave Studio, which is based on finite integration technique, is applied. The results are validated by measured results. The 3-dB beamwidth is 2.7°. A peak gain of 35.5 dB is predicted at 30 GHz. The reflectarray can cover Ka-band with sidelobe level below -20 dB for both E-plane and H-plane.