This paper reports on highly crystalline ZnO nanorods with different lengths and widths, grown using a microwave-assisted hydrothermal method at different irradiation times. The BET (Brunauer, Emmet, Teller) surface area increased while the lengths and widths of the ZnO nanorods were reduced with increasing irradiation time. The combination of photoluminescence (PL) and electron paramagnetic resonance (EPR) analyses elucidated that the defect-related properties affect the sensing response of the ZnO nanostructures. A broad visible emission band related to Zni and VO surface defects on the surfaces of ZnO nanorods was observed. The EPR studies revealed the ferromagnetic signal with a g-value of >2.0 for all ZnO nanostructures induced by Zni and VO surface defects. Gas sensors based on these ZnO nanorods exhibited a high sensing response to CO at 350 °C. Thus, our findings showed that the high response originates from surface defects (Zni and VO) formed at the surfaces of ZnO nanorods.