In this paper, a concept of partial structure reuse is presented and on this basis a 3.5/28 GHz shared-aperture antenna supporting two-dimensional (end-fire and broadside) steerable beams in MMW band is developed. The antenna consists of a 3.5 GHz planar inverted-F antenna (PIFA) and two 1×6 28 GHz substrate integrated DRA (SIDRA) beam-steerable arrays, in which partial structures of the PIFA are reused by the two MMW arrays. To improve the performance of PIFA, two narrow slots and one lumped capacitor are introduced to reduce its H-plane cross-polarization and enhance the operating bandwidth, respectively. To verify the design, antenna prototypes were fabricated and measured. Measured results show that the antenna can cover a dual-band of 3.28-3.67 GHz (11.2%), 26-29.7 GHz (13.2%) for end-fire and 25.9-29.7 GHz (13.8%) for broadside. The peak gains of 4.8 dBi, 10 dBi (end-fire) and 10.8 dBi (broadside) are obtained by the MW element and two 1×4 MMW arrays, respectively. Each of the MMW arrays can steer the beams from -50° to +50°, covering different parts of the space. The shared-aperture antenna has a compact size of 0.45×0.3×0.03 λ01 3 (λ01 is the free-space wavelength at 3.5 GHz), making it suitable for terminal applications.