We report the toxic gas sensing ability of ZnO Nanowire performed by using Density Functional Theory (DFT) based first principle approach. The ZnO Nanowire with rectangular morphology has been modeled and investigated for its energetic stability, analysed in terms of binding energy. The adsorption of a single molecule of carbon dioxide (CO2) gas on the fully relaxed rectangular ZnO Nanowire has been analysed through its dynamic response discussed in terms of conductance. The sensing ability has been evaluated further through adsorption energy, electronic band structure, density of states, Mullikan population and current-voltage analysis of the Nanowire. It has been observed that CO2 is physisorbed on the rectangular ZnO Nanowire via van-der-Waals interaction and relatively shows higher sensitivity of this morphology over other reports, defends this as a good candidate for CO2 sensing using ZnO structures.