This paper presents a novel highly directive ultra-wide band antenna for using in microwave imaging systems. Defected Ground Structure (DGS) is incorporated in the top and side of the ground plane to enhance the bandwidth impedance matching. Subsequently, for the further improvement in the bandwidth, gain and directivity staircase slot and square slot is etched at the bottom and top edge of the square patch plane. The low-frequency performance of the proposed antenna can be achieved by adjusting the gap between the ground and patch plane. The proposed microstrip antenna is fabricated on an FR4 substrate with dielectric constant of 4.3. The result shows that a directivity of 2.2-8.4dBi is achieved across a bandwidth from 1.43-8.92GHz. Furthermore, gain, directivity, radiation pattern characteristic, VSWR have also been analyzed at the different resonant frequency.