A uniplanar broadband balun is designed on SiO ₂ /Si substrate at sub-THz through this work. The structure is proposed for CPW to CPS transition establishing efficient on-wafer characterization environment for balanced antenna structures over the D-band. The balun is composed of a CPW linearly tapered into CPS and a broadband open termination of one side ground of CPW. The designed structure is analyzed and optimized through HFSS simulation. The designed model showed a bandwidth ratio of 55% at 140 GHz with insertion and return loss of 1 dB and 17 dB respectively. The results of the back-to-back structure are also presented.