The main objective of this paper is to design a very directive reflectarray antenna (RA) for biomedical imaging applications and breast cancer detection. The RA consists of a horn antenna as a feeder and a reflection surface based on double ring (DR) unit cell that achieves 350° phase range at a resonant frequency of 2.2 GHz. The designed RA is a pencil beam antenna with a high directivity gain of 19.6 dBi, small Half-Power-Beamwidth (HPBW) of and low Side-Lobe Level (SLL) of −15.9 dB compared to the horn antenna with a directivity gain of 6.5 dBi, HPBW of 109° and Side-Lobe Level of −12.3 dB. A fair comparison between the RA and the horn antenna is presented based on a 3D EM simulation with respect to Bio Tissue/Fat material box and human breast model as well. The designed RA achieves larger penetration depth and smaller power loss density/specific absorption rate (PLD/SAR) comparing with the horn antenna. Furthermore, it provides a higher resolution and a smaller angular scanning capability.