Imaging technologies have become more popular and widely used in various applications, including THz imaging systems. The THz spectrum offers specific characteristics for imaging, i.e., its penetration to dielectric materials and transmittance fingerprint of many materials, including biological tissues. Therefore, this advantage has been becoming potential for medical imaging applications as well. A device should be provided appropriately to transmit and receive the electromagnetic waves in developing a THz imaging system. The antenna technique adopted from microwave technologies can be considered to deal with this issue effectively. Among various antenna parameters, beamwidth is a primary consideration to obtain image quality of an imaging system. The array technique has provided a way to modify a single element antenna to have expected parameters of beamwidth as well as gain. This research study a horizontal array technique of a THz rectangular microstrip linear array antenna to get desired beamwidth antenna parameter and provide an antenna design for scanning purpose in a THz Imaging system. CST Microwave studio is used in designing and characterization to obtain optimum designs. By applying half-wavelength separation, the results show beamwidths in the same condition both on the vertical and horizontal beamwidth. The gain of two and three elements that modified into two sub-array horizontally are 9.0 dB and 8.8 dB, respectively. Horizontal beamwidths of the two and three array elements linear are 46.4 degrees and 39.0 degrees, respectively. By adjusting the beamwidth, these results are expected to improve the image quality of a THz imaging system.